PURPOSE: To obtain a stable optical coupling system having a good workability by fixing two optical fibers symmetrically to the center line, and fixing them so as to be condensed to one point of a photodetector through two spherical lenses or plano-convex lenses or GRIN lenses.

CONSTITUTION: A fiber fixing part 11 has optical fiber core inserting parts 1', 2', and nylon coated part inserting parts 6', 7' at symmetrical positions against the center line, and an optical fiber is inserted and stuck to them. A spherical lens holding part 12 for fixing spherical lenses 9, 10 for condensing a light from the optical fiber inserted through the inserting parts 1', 2' is provided on the tip of the fixing part 11. Also, a photodetector 16 for photodetecting a light from the lenses 9, 10 is provided on the tip of the holding part 12. In this case, the axes of the two optical fibers and the axes of the lenses 9, 10 are shifted so that the lights from the two optical fibers are condensed to one point of the photodetector 16. Such two input light photodetecting part is fixed by a pipe part 17 and a photodetector fixing ring 22, and a stable two input light photodetecting device having a good workability is obtained.