In a semiconductor laser device used as an optical time domain reflectometer, a polarizer is inserted in the path of the laser light. The plane of polarization of the polarizer is aligned to the plane of polarization of the emitted laser light, and the polarizer transmits the emitted laser light without attenuation. A reflected laser light from an optical fiber under test is a non-polarized light and receives a substantial attenuation by the polarizer. Thus, problems associated with reradiation echoes are prevented.