PURPOSE: To reduce the error of a tracking error signal by setting a hold voltage in which a tracking error original signal is sampled and held as the tracking error signal via an analog switch and a differential amplifier.

CONSTITUTION: A signal is inputted to a pulse signal generator 14 at the leading edge and the trailing edge of an output signal (c) waveform-arranged by a comparator 6, and a pulse signal (i) can be obtained from its output. The tracking error original signal (b) is inputted to a sample and hold circuit 15 controlled by the pulse signal (i), and the hold voltage (j) can be obtained by its output. Also, the hold voltage (j) is inputted to the analog switches 16 and 17 switched by the waveform-arranged output (c), respectively. And the output signals K and L of the analog switches 16 and 17 are differentially amplified by the differential amplifier 20, and furthermore, the tracking error signal can be generated via a low-pass filter 19. In such a way, it is possible to generate the tracking error signal with a few of errors.