A detection area is provided for detecting a light reflected off a first or second information recording medium and passed through an optical element, the first and second information recording medium are multiplayer disks each of at least two layers, the detection area contains a detection area (41) for detecting the first-order or higher diffraction light of a first-wavelength light and a detection area (42) for detecting the first-order or higher diffraction light of a second-wavelength light, the detection area (41) is disposed so as not cross over an area defined by the maximum range (38) of the zero-order light spreading of a light from a recording layer different from a recording layer subjected to recording or reproducing out of lights from the first information recording medium, and the detection area (42) is disposed so as not cross over an area defined by the maximum range (39) of the zero-order light spreading of a light from a recording layer different from a recording layer subjected to recording or reproducing out of lights from the second information recording medium.