PURPOSE: To enable multiple transmission of optical wavelength and to decrease coupling points and the coupling loss of light with simple constitution by using a plug type for a connector used t couple a light emitting part and a photodetecting part and a receptacle type at the incident and exit ends of the light subjected to wavelength multiplexing.

CONSTITUTION: The plugs 12-2, 12-3 provided at an optical multiplexer 11-1 are directly coupled to receptacles 14-2, 14-3 which are the exit ends of different wavelengths provided in a light emitting part 14. On the other hand, a receptacle 13 which is the exit end of the light multiplexed with different wavelengths by the multiplexer 11-1 is directly coupled to the plug 16-1 of a fiber cable 16. The 16-1 and the receptacle 13 are similarly directly coupled at the incident end of the demultiplexer 11-2 on the receiving side and the 12-2, 12-3 which propagate the different wavelengths demultiplexed by the optical demultiplexer are coupled to the receptacles 15-2, 15-3 in a photodetecting part 15. The transmission system for optical wavelength multiplexing having a decreased coupling loss of light is thus obtd.