Conserving land : protecting water

The present invention sets the position of each apparatus of a detecting system and an emitting system without using a reference disk in assembling an optical head. A light signal similar to reflected light of a reference disk is generated by a light modulator comprising a Faraday rotator for rotating a plane of polarization proportionally to the current by setting a solenoid coil having a large Faraday constant set around a piece of glass and electrifying the solenoid coil, a convex lens connected to a mechanism which moves along the optical axis in accordance with the applied current, a target section having a light detecting target with a groove formed on the surface and a light reflecting member set to the groove on the optical axis and connected to a mechanism moving along the radial direction of a disk in accordance with an applied current, so as to perform positioning by using the generated signal as a servo signal.