In the 21st century, the right of peoples to self-determination is firmly established as a norm of jus cogens under customary international law. Can the colonial rule be tolerated from the standpoint of the right of peoples to self-determination when a colonial ruler continues to exercise its administration of a part of the colonial territories by detaching it from the rest right before their independence in the 20th century of decolonization despite the world already is in the era of post-colonization? In international politics, this question may be interpreted as international liquidation of past that resolves, after a long time, a failed task due to the incomplete implementation of decolonization. In international law, it may be a matter of transitional justice, where development of history and changes in law are interrelated. In the Chagos Archipelago, the International Court of Justice (ICJ) reviewed the lawfulness of the continued administration by the United Kingdom of the Chagos Archipelago. The challenge facing the Court in this case is to choose between two conflicting values: the value of legal stability, which is the application of rules with definitive content at a given time; and the value of legal rationality to accommodate structural changes to legal issues of historical character. In this case, the Court could be understood to take the legal reasoning of applying the findings and holdings of applicable law, which appears to take the latter approach. First, the Court adopted an evolutionary approach that embraces the “evolution of the law” in determining applicable law. Second, the Court deduced the obligation to respect territorial integrity of a colonial country from the concept of the right to self-determination that is easier to prove general practice. The Court has identified and applied customary international law in a manner appropriate to derive the rights of colonial peoples to self-determination and the obligation of territorial integrity by colonial rulers as its corollary. First it interpreted State’s act of adopting resolution as State practice so that the type of State practice that underlies the general practice of self-determination can be expanded. Second, in finding established general practice of the States, the Court took multilateral approach which is more appropriate in making the majority countries obligate minorities, instead of the traditional State-centered consensus-based approach. Third, it avoided officially dealing with the rule of persistent objector, which is a safeguard for minority countries against the majority-generated customary international law. The Court s approach in this case has resulted in the identification and application of laws at the international level to realizing transitional justice. At the same time, however, it caused partial lessening of the standards and procedures in confirming customary international law.