To accelerate a normalization operation by multiplying the output signals of an offset means for supplying a DC offset to the output of a first multiplication means and the output of a delay means for supplying delay time equal to the delay time received by first input signals to second input signals.

The sum output R0 signals of a main beam are clamped for an APC period when a laser diode is OFF by a clamp amplifier 1 and a sample- and-hold circuit 2, an offset voltage 6 is supplied to the output V2 of a first multiplier 4 in a DC shifter 5 further and it is supplied to a second multiplier 7. In the meantime, the difference output CMO signals of a side beam are offset and clamped for the APC period by the clamp amplifier 8, the sample-and-hold circuit 9 and a clamp reference 10. Delay equal to the operation delay time of the first multiplier 4 is supplied to the clamped value in a delay circuit 11 and it is supplied to the second multiplier 7. Then, the relative time difference of two input Vx and Yy in the second multiplier 7 is turned to 0.