Taisei Kōki (大成光機) is a Japanese camera maker that was active after World War II.
The three companies Fuji Kōgaku, Katsuma Kōgaku and Taisei Kōki are probably related. All three used the Terionar lens name, and all three have similar logos, showing a cemented doublet lens scheme, with a protuberance on top of the logo, maybe an allusion to the Mount Fuji. In the case of Taisei Kōki, the logo used on the Welmy range is written TAISEI WELMY.
Various Japanese websites say that Fuji Kōgaku became Taisei Kōki, but this page at Japan Family Camera more precisely says that the company was split in two parts: Taisei Kōki and Katsuma Kōgaku, with the latter one keeping the Lyra brand name (Fuji Kōgaku and Katsuma both used the brand name Lyra on various cameras). However there are records of Katsuma Kōgaku dated as early as 1932 (it contributed to found the Tōkyō Kōgaku company, later Topcon), so this cannot be completely accurate.