It is intended to provide an optical disk apparatus which detects a light amount greater than zero even when used in conjunction with an optical disk substrate having a large birefringence, so that it is possible to properly read a signal without errors and properly perform optical disk controls. The optical disk apparatus includes: a light source for emitting light; an objective lens for converging the light onto a signal surface of an optical disk; a polarized beam diffraction element for diffracting the light reflected from the optical disk; a photodetector for detecting the light diffracted from the polarized beam diffraction element; and a wavelength plate disposed between the optical disk and the polarized beam diffraction element. The wavelength plate has a two-dimensional array of a plurality of birefringent regions including first and second regions, the first and second regions differing in birefringent phase difference and/or optic axes from each other, and the plurality of birefringent regions including the first and second regions cause the light to have different polarization states.