International conventions on ship safety and protection of the marine environment are implemented from two positions: the flag State and the port State. This article explains the role of each and how they differ.

The IMO (International Maritime Organization), which develops these conventions, has no power to enforce them itself. It is individual countries that actually apply them.

1.1.1 What Is a Flag State?

A flag State is the State whose flag a ship flies, in other words, the State that has granted the ship its nationality.

Under the United Nations Convention on the Law of the Sea (UNCLOS), each State sets the conditions for granting its nationality to ships, for registering ships, and for the right to fly its flag. Ships have the nationality of the State whose flag they are entitled to fly, and there must be a genuine link between that State and the ship.

A ship may sail under the flag of one State only. On the high seas, it is subject to the exclusive jurisdiction of that flag State, except in cases provided for in treaties or in UNCLOS. A ship that uses the flags of two or more States according to convenience may not claim any of those nationalities and may be treated as a ship without nationality.

1.1.2 Responsibilities of the Flag State

UNCLOS requires each flag State to effectively exercise its jurisdiction and control in administrative, technical, and social matters over ships flying its flag. The main elements are as follows.

• Maintain a register of ships and apply its internal law to each ship and to its master, officers, and crew
• Take measures to ensure safety at sea with regard to the construction and equipment of ships, the manning and training of crews, the use of signals and communications, and the prevention of collisions
• Have each ship surveyed by a qualified surveyor before registration and at appropriate intervals thereafter
• Ensure that the master and officers have appropriate qualifications and that the crew is appropriate in qualifications and numbers for the ship

For IMO conventions as well, it is the flag State that applies the conventions to its own ships and crews. When an offence occurs on the high seas, the responsibility for imposing a penalty also rests with the flag State.

IMO Member States have the primary responsibility to establish and maintain a system to fulfil their obligations as flag, port, and coastal States. The IMO has established the III Code (IMO Instruments Implementation Code), which shows how Member States should implement the conventions, and uses it as the standard for checking implementation through the IMO Member State Audit Scheme (IMSAS). Audits became mandatory in January 2016, and each Member State is audited every nine years.

1.1.3 What Are Flags of Convenience?

A ship that a shipowner has registered in another country for convenience in order to reduce costs is called a flag-of-convenience ship. Examples of such registries include Panama, Liberia, Malta, and Cyprus.

Shipowners register ships in these countries because of lighter tax burdens, less strict operational regulations concerning seafarers, and the ability to employ foreign seafarers at lower wages. The shipowner then either operates the ship itself or charters it out to other companies.

On the other hand, flag-of-convenience ships are considered to have problems in terms of safety, environmental protection, and security. For this reason, some countries have introduced a second registry system aimed at securing the cost competitiveness of ships under their own flag and preventing the growth of flag-of-convenience ships. Its main measures include special arrangements for crewing with national seafarers and reduced ship registration fees, and it has been introduced in countries such as France, Norway, Denmark, and Germany.

1.1.4 What Is a Port State?

A port State is the State in which the port a ship enters is located. A port State can inspect foreign ships in its ports. This inspection is called Port State Control (PSC).

Periodic surveys of ships are originally carried out by the flag State. However, in some countries, surveys by the flag State are not carried out sufficiently, and substandard ships that do not meet convention standards continue to operate. PSC was established as a measure against such ships.

In PSC, inspectors of the port State board foreign ships to verify that the condition of the ship and its equipment complies with the requirements of international regulations, and that the ship is manned and operated in compliance with the conventions. Examples of problems include equipment failure, insufficient crew, and overloading.

An inspection begins by checking that valid certificates and other relevant documents are on board. If there are clear grounds for believing that the condition of the ship or its equipment does not correspond substantially with the particulars of the certificates, or that the master or crew are not familiar with essential shipboard procedures, a more detailed inspection is carried out.

If deficiencies are found, the port State requires them to be rectified and may delay the ship’s departure or detain the ship until the necessary corrective actions are taken. The ship may also be targeted for future inspections. At the same time, all possible efforts must be made to avoid a ship being unduly detained or delayed.

When an offence occurs within its jurisdiction, the port State can either take proceedings under its own law or give details of the offence to the flag State so that the flag State can take appropriate action. UNCLOS also provides that a State that has ascertained that a ship in one of its ports is in violation of international rules on seaworthiness and thereby threatens damage to the marine environment shall, as far as practicable, take administrative measures to prevent the ship from sailing.

The main conventions used as the basis for PSC in Japan are as follows.

Table 1.1-1 Main Conventions Used as the Basis for PSC

Abbreviation	Convention
LL	Load Lines Convention
SOLAS	International Convention for the Safety of Life at Sea
MARPOL	International Convention for the Prevention of Pollution from Ships
STCW	International Convention on Standards of Training, Certification and Watchkeeping for Seafarers
MLC	Maritime Labour Convention
BWM	Ballast Water Management Convention

1.1.5 Regional Cooperation in PSC

PSC is carried out through regional cooperation among countries. Coordinating inspections of ships that call at several countries in the same region makes it possible to focus inspections on substandard ships while avoiding repeated inspections of the same ship.

The Paris MoU was adopted in Europe in 1982, and the Tokyo MoU was adopted in the Asia-Pacific region in 1993. There are currently nine regional PSC regimes, made up of eight Memoranda of Understanding (MoUs) and one Agreement.

Table 1.1-2 Regional PSC Regimes

Regime	Region
Paris MoU	Europe and the North Atlantic
Tokyo MoU	Asia and the Pacific
Acuerdo de Viña del Mar	Latin America
Caribbean MoU	Caribbean region
Abuja MoU	West and Central Africa
Black Sea MoU	Black Sea
Mediterranean MoU	Mediterranean Sea
Indian Ocean MoU	Indian Ocean
Riyadh MoU	Persian Gulf

In addition, the United States Coast Guard maintains its own PSC regime. Some countries belong to more than one regime.

The Tokyo MoU was signed on 1 December 1993 and took effect on 1 April 1994. Its Secretariat was established in Tokyo on 15 March 1994. The committee made up of its member Authorities works on matters such as harmonizing procedures and practices for inspection, rectification, and detention.

In Japan, a system of dedicated PSC officers was introduced in fiscal year 1997, and these officers are assigned to District Transport Bureaus and other offices across the country.

1.1.6 Differences Between Flag States and Port States

The differences between flag States and port States can be summarized as follows.

Table 1.1-3 Differences Between Flag States and Port States

Item	Flag State	Port State
Position	The State that has granted the ship its nationality	The State in which the port the ship enters is located
Scope	Ships flying its flag and their crews	Foreign ships in its ports
Main roles	Ship registration, application of national law, periodic surveys, ensuring crew qualifications	On-board inspections through PSC, requiring rectification
Response to violations	Imposes penalties for offences on the high seas	Delays departure or detains the ship until rectification; can take proceedings under its own law or report to the flag State

The primary responsibility for ensuring that a ship meets the required standards rests with the flag State. PSC by port States backs up implementation by the flag State and is regarded as a “second line of defence” against substandard ships.

About This Article

References

• United Nations Convention on the Law of the Sea, Articles 91, 92, 94, and 219

Sources

• International Maritime Organization, “Conventions”
• International Maritime Organization, “Port State Control”
• International Maritime Organization, “Member State Audit Scheme”
• Ministry of Land, Infrastructure, Transport and Tourism, “What Is PSC?” (in Japanese)
• Maritime Bureau, Ministry of Land, Infrastructure, Transport and Tourism, “Glossary” (in Japanese)
• Tokyo MOU, “Organization”

(Last accessed: October 15, 2026)


*This article was compiled based on the author’s understanding and practical knowledge, with reference to the above literature.