The laser light source, the PBS, and the reference light mirror are arranged in such a manner as to simultaneously satisfy Lsig+LBS≠s (Lld/m) (s: positive integer, m: positive integer), Lref+LBS≠t (Lld/m) (t: positive integer), and u(Lld/m)−(ΔL/2)≦Lsig−Lref≦u(Lld/m)+(ΔL/2) (u: integer), where Lld represents an in-vacuum internal resonator length of the laser light source, LBS represents an in-vacuum-converted optical path length of a laser beam between the emission end surface of the laser light source and the PBS, Lsig represents an in-vacuum-converted optical path length of signal light between the PBS and the reflecting unit of the optical disk, Lref represents an in-vacuum-converted optical path length of reference light between the PBS and the reference light mirror, and ΔL represents an interference permissible optical path length of the laser beam.