PURPOSE: To decrease the time required for locking control by increasing/ decreasing largely at first a gain from the initial gain to an object gain and increasing/decreasing gradually by 1/2 of the preceding gain.

CONSTITUTION: The initial gain of a variable gain amplifier is set to the maxi mum or minimum gain and the gain is controlled by a control circuit discretely at a prescribed interval over a maximum of 2N stages. A discrimination circuit 2 inputs the output pulse S2 of the amplifier 1, the voltage is compared with a reference voltage, counts the number of output pulses in excess of the refer ence voltage for a prescribed time and outputs a discrimination signal representing the quantity through the comparison with the set value. The control circuit 5 outputs a signal to a switch drive circuit 4 so that the gain of the amplifier 1 is increased/decreased gradually as 2N-1, 2N-2...2, 1 from the initial gain toward the object gain. The circuit 4 controls the feedback quantity accord ingly. Thus, the locking control time is reduced and the gain control of 2N stages is finished by N times.