PURPOSE: To prevent generation of error and deterioration of picture quality by providing a variable time constant type integration device as a primary lag element after a smoothing circuit of a rectifier circuit obtaining an output of a control circuit.

CONSTITUTION: An output to be controlled of a control circuit comprising a photocoupler whose impedance is changed by a control current and an L type attenuator paired with the said photocoupler is fed to an amplifier 2 branching the input signal into two, one of two outputs of the amplifier 2 is fed to the rectifier circuit 4 via an auxiliary amplifier 3. The rectified output of the rectifier circuit 4 is fed to the integration circuit 5 as the primary lag element comprising a capacitor C1, a variable resistor R1 for deciding the control time and a DC amplifier having a sufficiently high amplification factor, and the output of the primary lag element 5 is fed to a voltage-current converter 6 to obtain a current driving the photocoupler of the control circuit 1.