PURPOSE: To enable transmission of a light signal without receiving the influence of a change in ambient temp. by sandwiching plural pieces of optical fibers in a case by two fixing members having approximately equal coefft. of thermal expansion.

CONSTITUTION: A light emitting element 10 and a photodetecting element 11 are connected to three optical fibers 3, 4, 5 coupled by a three-forked part 2 in a case 1. The optical fibers are sandwiched by a metallic member 17 and glass member 18 having roughly equal coefft. of thermal expansion and are fixed to the case 1 by an electrostatic joining method. Since the difference in the coefft. of thermal expansion between fixing members 17, 18 and the optical fibers is small, the mechanical strain of the optical fibers by a change in temp. is decreased and the influence of ambient temp. on the performance for transmitting signal is obviated.