PURPOSE: To provide an optical device which can be reduced in size, mass-produced, and hardly requires adjustment, and can maintain the efficiency of returning light detecting power.

CONSTITUTION: An optical device is provided with a light emitting section composed of a laser light source 3, reflecting surface 2 which is provided near the light emitting section and to which directly returning light 12 from the light emitting section is made incident, reflecting means composed of a reflecting mirror 4 and counterposed to the reflecting surface 2, a light receiving section composed of photodiodes PDA, PDB, PDC, and PDD counterposed to the reflecting means 4 and arranged in the same plane as that of the reflecting surface 2. The optical device can detect RF signals, focus error signals, and tracking error signals required at the time of reading optical recording media even when the device is not provided with such a means as the beam splitter, etc., to split the emitted light and returning light.