Election type

Public Offices Election Law of 1950 actually allowed voting from abroad (Art. 49- 2). However, according to information in the following publication, the external voting was not practices up until 1998: 

The Political Participation and Constitution in Japan by Shigenori Matsui: "On the other hand, Japanese citizens living abroad who had no address in Japan were deprived of any opportunity to participate in elections before 1998. Even after an amendment of Public Office Election Act in 1998, overseas voters were allowed to participate only in the proportional representation elections (page 11). The government finally amended the Public Office Election Act in 2006 to allow overseas voters to participate in election districts (page12)."

See also this article for detailed explanation of the external voting reforms in Japan: The voting rights of Japanese citizens living abroad

Ministry of Internal Affairs and Communications, About the overseas election system [in Japanese], accessed 7 November 2021