There are comprised a laser light source for outputting a light beam, an optical disk having a recording/reproducing surface and a transparent substrate on the surface, an objective lens for gathering the light beam on the optical disk, a relay lens system comprising one or more lenses inserted between the laser light source and the objective lens, means for moving one or more lenses in the relay lens system toward its optical axis so as to correct sphere aberration in accordance with a thickness of the transparent substrate of the optical disk, and means for sustaining a diameter of an incident light on the objective lens when one or more lenses among the relay lenses move.