Light beams emitted from a light source (11) and returned light beams reflected off magneto-optical disks (7a), (7b), (7c), (7d) are transmitted by using polarization preserving optical fibers (15a), (15b), (15c), (15d). Phase difference generating means (16a), (16b), (16c), (16d) are disposed in the optical paths of the light beams, and the phase differences of electric field vibration components occurring in the light beams due to double refraction of polarization preserving optical fibers (15a), (15b), (15c). (15d) are offset by phase differences produced by phase difference producing means (16a), (16b), (16b), (16c), (16d), or the phase differences are adjusted to be multiples of pi .