PURPOSE: To use a plastic-made optical disc to be easily deformed and to position an optical system easily by using transmitting light prevented from the influence of the inclination of a disc surface to obtain a tracking error signal.

CONSTITUTION: The optical disc 30 is constituted of a transparent substrate 31 and a translucent reflecting film 32. The reflecting film 32 is an essential factor and about 5W50% transmissivity is preferable. A general metallic thin film or a chromatic film is used as the reflecting film. Guiding grooves or information tracks are formed as uneven shapes on the reflecting film. A laser beam is condensed on the translucent reflecting film 32 through a collimator lens 34, a condenser lens 35, etc. A part of the irradiated laser beam is reflected and a part is absorbed, but a part is transmitted and made incident to a photodetector having photodetecting areas A', B'. If it is defined that the photodetecting variables of the areas A' B' are a' and b' respectively, a differential amplifier 38 calculates a'-b', so that a tracking error signal can be obtained. The positioning of the optical system can be easily attained by using transmitted light prevented from the influence of the inclination of the disc surface.