To discriminate a disk in a short time, a calculating part 16 measures an amplitude of one of a focus error signal and a tracking error signal, which are output from a differential signal generating part 15, several times, and outputs the measured amplitudes to a controller 20. The controller 20 compares the input amplitudes with threshold values stored in a memory 21, respectively, and determines whether or not an amplification gain of one of the focus error signal and the tracking error signal is changed. If it is determined that the amplification gain of one of the focus error signal and the tracking error signal is changed, the controller 20 changes the amplification gain of one of the focus error signal and the tracking error signal to an amplification gain represented by one of the threshold values and discriminates the kind of an optical disk based on the changed amplification gain.