HMW demodulation is effected more reliably with a simple configuration. A reproduced wobble signal is caused to pass through a BPF previously set to a center frequency 2fwob, to thus enhance a second harmonic component. The wobble signal having passed through a BPF is multiplied by a second harmonic signal. Subsequently, a peak envelope signal and a bottom envelope signal of the multiplied wobble signal are extracted. The peak envelope signal is discriminated from the first preset threshold value. A result of this discrimination becomes a signal used for detecting an HMW[1] wobble waveform. Similarly, a bottom envelope signal is discriminated from a preset second threshold value. The result of discrimination becomes a signal used for detecting an HMW[0] wobble waveform.