PURPOSE:To reduce the frequency of an addition signal and to suppress heat generation and noise by selecting the outputs of a pair of detectors through a switch, detecting the signal level with approximately twice the frequency of an addition signal outputted from the switch and controlling the gain of outputs from the detectors in accordance with the signal level. CONSTITUTION:A tracking error signal is formed from a difference between the outputs of a pair of detectors 1, 2 and an actuator 11 is driven by a signal obtained by adding the addition signal with the prescribed frequency to the tracking error signal. The outputs of a pair of the detectors 1, 2 are alternately selected through the switch, the signal with approximately twice the frequency of the addition signal outputted from the switch is formed and its level is detected. The gain of the detectors 1, 2 are controlled correspondingly to the signal level. Consequently, an information detecting point follows tracks while traveling zigzag in accordance with the frequency of the addition signal. Thus, noise or heat generation due to the addition signal can be reduced.