Laser light emitted from the wavelength-locked GaN semiconductor laser is collimated through a collimation lens, led through a polarization beam splitter and a 1/4 wavelength plate, and converged by a focus lens so as to be radiated on pits formed in an optical disk medium. The signal light from the optical disk medium is collimated by the focus lens, and has its polarization direction turned by the 1/4 wavelength plate by 90.degree. relative to its polarization direction before being returned from the optical disk medium. As a result, the signal light is reflected from the polarization beam splitter so as to be converged on the optical detector by the focus lens.