Taisei Kōki (大成光機) is a Japanese camera maker that was active after World War II.
It is probably related to Fuji Kōgaku. Various Japanese websites say that Fuji Kōgaku changed its name to Taisei Kōki after the war, but this page at Japan Family Camera more precisely says that Fuji Kōgaku was split in two parts after the war: Taisei Kōki and Katsuma Kōgaku. This cannot be exact because there are earlier records of the existence of Katsuma, but Katsuma could have bought the Lyra brand name and other assets to Fuji Kōgaku after the war, while the rest of the company became Taisei Kōki.
Indeed the logo used on the Welmy range is very similar to the Fuji Kōgaku logo: the words TAISEI WELMY are written in a cemented doublet lens design with a protuberance on top, that was an allusion to the Mount Fuji on the Fujikō logo. Taisei Kōki also used the Terionar brand name on some lenses, a brand name that was already used by Fuji Kōgaku.