In multilayer optical discs and high-speed optical discs, the amount of reproduction light per unit time greatly decreases and the reproduction signal quality (S/N) significantly drops due to the low effective reflectivity and the short read time of the medium. These problems are solved by causing reflected signal light from the optical disc and reference light, which is separated from the same light source and introduced into a detector without being shone onto the optical disc, to interference with each other on the detector. Detector outputs having four different interference states are simultaneously obtained, the interference states being displaced at intervals of 90° in terms of the phase relationship between the reference light and the signal. Based on a operation of the four detector outputs, a reproduction signal can be obtained that is stable at all times and amplified with high quality, even when there is an optical path length variation due to disc undulations.