In 1947 Japan adopted a new Constitution replacing the old Meiji Constitution of 1899. Disregarding from the German experience where democratic German lawyers were authorized to draft their Grundgezetz of 1949, in Japan Constitution was drafted by American military lawyers.
Based on Westminster system, Japanese Constitution introduced the system of division of powers, human rights and rule of law. The most important was introduction of constitutionality review.
Since then, the Supreme Court of Japan has established constitutionality review as a current practice. The Japanese Constitution of 1947 is a very rigid one.
There has been no amendment during seventy years of its existence.