PURPOSE: To obtain a semiconductor laser module built in an optical isolator for bidirectional communication of simple constitution capable of receiving signal light of a backward direction by using double refractive elements for the polarizing element of the optical isolator.

CONSTITUTION: The forward direction signal light (f) emitted from the semiconductor laser 2 is made incident on the optical isolator 5a collimated by a first lens 3 and the incident light on the first double refractive element is separated into an ordinary ray f1 and an extraordinary ray f2, which are made incident on the Faraday rotator 9. The rays are, thereupon, respectively rotated in plane of polarization by 45° and are made incident on the second double refractive element 12. The extraordinary ray f1 advances rectilinearly and the extraordinary ray f2 is further changed in its position and is emitted. The backward direction signal light (b) emitted from an optical fiber 6 is made incident on the optical isolator 5b collimated by a second lens 4. This light is processed similarly for the light in the forward direction. The ordinary ray b1 acts as extraordinary light and the extraordinary ray acts as ordinary light in the first double refractive element 11 and, therefore, the light does not return to the semiconductor laser 2 and both of the ordinary ray b1 and the extraordinary ray b2 are received in a photodetector 10.