PURPOSE:To eliminate the DC offset of a servo signal which is generated due to stray light generated inside an optical head and obtain a stable error signal. CONSTITUTION:A conical part 10 which is inclined for the 11 light axis of a second surface of a detection lens 10 is provided on the second surface and stray light 17 generated in an optical head entering the detection lens 10 with an angle of field is totally reflected by a conical part 10c on the second surface. thus guiding the stray light in a direction which is different from a photo detector and suppressing the stray light entering the photo detector.
