To provide an optical module in which the light utility efficiency is improved, a noise due to a stray light and a returning light is decreased.

Forwarding lights L outputted from a semiconductor laser element 1 are made incident on a diffraction element 3 as a P-polarized light and diffracted. The diffraction element 3 has a low diffraction efficiency for the P-polarized light and most portion of the forwarding light becomes a zero- order diffraction light La. A returning light M reflected on a recording face of an optical disk 6 returns as a circular polarized light having a direction reversed from that when the recording face of the optical disk 6 is irradiated, and is transformed into an S-polarized lights with a 1/4-wavelength plate 4. The returning light M transformed into the S-polarized light is diffracted with the diffraction element 3. The diffraction element 3 has a high diffraction efficiency for the S-polarized light and most portion of the returning light M becomes a positive 1st order diffraction light Mb and a negative 1st order diffraction light Mc. The returning light M diffracted as the negative 1st order refraction lights Mc is made incident on a light receiving element 7.