PURPOSE: To decrease the loss of the light and to improve the reflecting ratio by providing the means to photodetect the light source to irradiate the light to the optical disk and the reflecting light of the irradiating light through the deflecting plate.

CONSTITUTION: The optical disk has a pair of opposing substrates 3 and substrates 7, one side opposing substrate 3 is at least translucent. Further, inside the pair of substrates, the disk has a pair of electrodes 4 and 6, an opposing electrode 4 has the translucent characteristic and the electrode 6 has the reflecting characteristic. By the optical beam from a semiconductor laser 12, light 16 is made incident through a half mirror 2 on a converging optical system and through the correction (auto tracking device) 11 of the position on an optical disk 10, further, reflected from the optical disk 10 as light 16', the optical path is separated by the half mirror 2 and reaches a photodetecting sensor 9 through a polarizing plate 8. thus, the deflecting plate is arranged at the photosensor part, the plate is one, and therefore, the loss of the light can be minimized and the reflecting ratio can be highly kept.