A Michelson interferometer 1 of a thickness measuring device 100 comprises two optical cables 4 and 5 connected by an optical coupler 3, and the optical cables 4 and 5 are formed by polarization optical fibers 41 or the like, connected by the optical connectors 42 or the like. The optical connectors 42 or the like, are adjusted in angle and position by rotating the optical fibers 41 or the like, to be connected around the optical axis.