PURPOSE: To increase the detecting width and at the same time to improve the detecting accuracy, by providing a pair of reflectors having light transmitting parts between a light source and an optical device to detect both reflected light and transmitted light as well as a focus shift and a tracking shift, respectively.

CONSTITUTION: Each of reflectors 12 and 13 has a reflecting part and light transmitting part 22. A projection pattern is circular on a surface rectangular to an optical axis and has a shape equal to the section a luminous flux 10a. When an image forming surface 15 is set at a proper position 15a, detectors 17 and 19 detect no light. However only, the detector 17 detects the light at a position 15b and only the detector 19 does at a position 15c, respectively. Then the light made incident to a detecting surface 30 of detectors 17 and 19 respectively is detected through a photoelectric conversion. The surface 30 is divided into two surfaces 31 and 32 along a center line 33 to detect also a tracking shift. In other words, a tracking shift is detected from the error between detecting signals of surfaces 31 and 32.