The Constitution of Japan was promulgated on November 3, 1946, and went into effect on May 3, 1947. The first session of the National Diet was convened on May 20 of that year. That session marked the birth of the National Diet, replacing the Imperial Diet which had been created in 1890 under the Meiji Constitution and served for over 56 years.
The National Diet :The Design Provided in the Constitution Powers of the National Diet