PURPOSE: To provide an extremely precise and high speed servomechanism with a simple configuration.

CONSTITUTION: Signals from a differentiating circuit 3 are supplied to a retriggerble single stable multivibrator 30 of 11 clock reversing time. The output of the multivibrator 30 is supplied to a retriggerble single stable multivibrator 31 of four frame reversing time. Furthermore, lock indicating signals from a synchronization separating circuit 5 are supplied to reversal enable terminals of the multivibrators 30 and 31. The signals from the multivibrator 31 are supplied to a comparator circuit 33 through a resistance 32. An arbitrary potential from a voltage dividing circuit 34 is supplied to the comparator circuit 33. The output of this comparator is supplied to a NAND circuit 35 and simultaneously supplied to a NAND circuit 37 through an inverter 36 and the outputs of the NAND circuits 35 and 37 are supplied to the bases of transistors 21 and 22 and 23 and 24, respectively.