PURPOSE: To accurately detect a signal by arranging a one-direction angle displacement generating member on the optical path through which the reflection light from an optical information recording medium is led to a signal detecting optical system.

CONSTITUTION: In a signal detecting optical system 16, a prism 18 is arranged on the optical path where the reflection light from a magneto-optical disk 15 which is reflected by a beam splitter 12 passes a λ/2 plate 17. This prism 18 functions to displace the reflection light at an angle only in one direction in accordance with defocusing. A focus error signal is detected in positions before and after a light spot P where light is condensed through a polarizing beam splitter 19 by condenser lenses 20 and 22. The beam sectional form is always a perfect circle in any position in the optical axis direction in the case of the in-focus state, and the focus error signal does not include the offset even in the case of the occurrence of positional deviation in the optical axis direction, and therefore, the signal is accurately detected.