On the 10th of July 2003, a bill cleared the Japanese Diet and was promulgated six days later ironically under the name and with the seal of the Emperor which is often said to represent the ‘patriarchal symbolic system’ of Japan. In defiance of the long-accepted idea that Parliament “can do everything except make a woman a man, or a man a woman”, this law enabled people with ‘gender identity disorder’ to legally change the sex registration on the family register (koseki). In this article we offer a brief description of this legislative process and a certain normative argument on it. Firstly, we examine the very concept of transgender, transsexual and gender identity disorder from medical and sociological viewpoints. In addition, the history and environment surrounding transgender in Japan is outlined here. Secondly, we take a look at the legislative process itself on its two phases formal and informal. The formal phase is concerned with the public/visible procedures mainly in the Diet, and the informal one with the ruling party's internal/nvisible examination. Standing on these analyses based on social and political reality, we go further to examine the contents of this law. Compared with similar laws in such countries as Sweden, Germany, Italy…, this new Japanese legislation is more severely termed, notably in that only people without children are allowed to change their registered sex. At the same time, the law contains a proviso that it is to be reviewed and opened to revision in 2007 (three years after its enforcement). Though legal philosophers have traditionally paid less attention both theoretically and practically to legislature than to judiciary, this epoch-making legislation and its process in Japan seem to offer us a meaningful insight into the former.