PURPOSE: To perform compensation of asymmetry with high accuracy, by detecting inversion interval of positive and negative polarities, and making up-count with one detected output and down-count with another detected output.

CONSTITUTION: The revolving speed of a disc is increased, the maximum inverting interval of a signal S0 is shorter than a specified value, and an output of a counter 21 goes to "1". In this case, a gate circuit 74 is set, a clock pulse from a clock generating circuit 50 is applied to the down-count terminal of an up/down counter 61 and the count value is decreased. Thus, an output voltage of an output processing circuit 62 is lowered and revolving speed of a motor is reduced. When a clock pulse is obtained if the output of a D flip-flop circuit 40 goes to "1", this clock pulse is applied to an up or down count terminal of an up/down counter 71, then the control is provided to compensate the phenomenon of asymmetry.