To obtain an optical multiplexer/demultiplexer module manufactured to be compact at low cost, made stable to environmental fluctuation and coping with high-output incident light and desirably realizing a small amount of light loss and high performance.

A 1st linearly polarized light beam 14 which is emitted from a 1st optical waveguide 30 and made parallel beams by a 1st lens 24 and a 2nd linearly polarized light beam 16 which is emitted from a 2nd optical waveguide 32 and made parallel beams by a 2nd lens 26 and whose optical path is changed by a prism 12 are made incident on the incident surface 18 of a uniaxial double refraction crystal 10 with separation width D, and multiplexed on the emitting surface 22 of the crystal 10. The multiplexed light 20 is condensed by a 3rd lens 28 and made incident on a 3rd optical waveguide 34.