In addition to researching the national (domestic) law of Japan, you will need to also research Japan's obligations under international law.
Article 38 of the Statute of the International Court of Justice sets out the sources of international law:
The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
The most important source of international law are international conventions, also known as treaties or international agreements. This guide will provide information on finding treaties where Japan is a party.
Treaty research requires finding a full-text copy of a treaty or international agreement, then checking the status to see if it has entered into force, whether a party has ratified it, or if there have been any amendments.
Copies of treaties and status information can generally be found on the depositary's website. A depositary is a government or organization to which the treaty is entrusted. Responsibilities include accepting all notifications and documents relating to the treaty, examining whether formal requirements are met, and notifying the parties of relevant acts.
The example above indicates that Japan is a party to this convention after acceptance. Note that the status page provides access to the full-text treaty (both the certified true copy as well as the version as published in the United Nations Treaty Series).