To reduce the influence of the noise of a returned light beam in an optical pickup without accompanying the increase of the cost and a large-sized device due to the increase of the number of parts.

A region in which the noise of a returned light beam increases exists at the position of the integral multiple of NL from the light-emitting surface SA of a semiconductor laser 10 for inner resonator length L and an effective refractive index N in the laser. On the other hand, the reflection plane of a disk 16 is displaced by surface wobbling. When the wobbling range puts within a node region WB interposed by noise increasing regions WA, the noise of the returned light beam is reduced. When this is represented by a numerical formula and an arbitrary integer is made to (m), since the outer resonator length LA preferably exists between mNL and (m+1)NL; the LA becomes mNL< LA<(m+1)NL.