PURPOSE: To obtain a highly reliable servo circuit by providing an A/D converter which makes a difference signal be an analog input, and besides, makes a sum signal be a reference input, and outputs the digital signal of an offset binary type, as a normalization processing means.
CONSTITUTION: It is assumed that the outputs of IV converters 2, 3 are positive, and an output offset reference value V 0 >0. The sum signal Vx=x+V 0 and the difference signal Vy=y+V 0 of operational amplifiers 4, 5 are obtained, and the A/D converter 17 compares between the signal x+V 0 and ±y+V 0 by a width, corresponding to its bits number and encodes them. An output signal DB comes to be a full code in a condition that a light spot concentrates to a detector 1a (x=y), and comes to be a zero code if a light receiving quantity is zero (x=-y), and it comes to be code information that the value (x) is normalized by (y). Besides, because if x=0, the code information in the signal DB has the offset of 1/2 full code, it comes to the offset binary type. When a D/A converter 19 performs a focusing drive 8 so that an operating point comes to zero by outputting the signal Ax, and performs a tracking drive 15 by the signal Az, it can servo-control a device at a high reliability.