To detect a reflected light only from a recording/reproducing target layer by eliminating stray lights from other layers.

An optical system is provided with: a semiconductor laser 41 dedicated to the L0 layer light condensing of an optical disk medium 80; a photodetector 44 for receiving a reflected light from the L0 layer; a semiconductor laser 51 dedicated to L1 layer light condensing; and a photodetector 54 for receiving a reflected light from the L1 layer, wherein the light receiving amount PD00 of the photodetector 44 and the light receiving amount PD01 of the photodetector 54 are measured during the driving of the semiconductor laser 41, the light receiving amount PD10 of the photodetector 44 and the light receiving amount PD11 of the photodetector 54 are measured during the driving of the semiconductor laser 51, and based on these measured values and a plurality of constants calculated by optical designing, the received amounts of the reflected lights only from the L0 and L1 when the semiconductor lasers 41 and 51 are driven are calculated. Thus, the reflected light only from the recording/reproducing target layer is detected by eliminating stray lights from the other layers.