With the tracking servo loop in an open state, when the beam spot S1 moves from one record track T1 to an adjoining record track T2 as shown in FIG. 3A, the tracking error signal represents such a waveform as sinusoidal waveform as shown in FIG. 3B. The level of the tracking error signal is proportional to the offset of the beam spot S1 from the record track T and the zero-crossing points correspo