PURPOSE: To remove hunting especially on a small diaphragm side without deteriorating an operability by changing the responding speed of a diaphragm in response to a stop value.

CONSTITUTION: A luminance signal Y outputted from a camera signal processing circuit 4 is inputted to an integration circuit 6 and integrated so that a control signal for obtaining an appropriate exposure according to the luminance state of a screen can be generated. Then, the signal is compared with a reference value 8 by an arithmetic part 7, and the difference signal is generated. The difference signal is supplied through multipliers 13 and 14, and a driver 9 to an actuator 10 as an exposure control signal, and an iris 2 is controlled so that the difference signal can be constant. On the other hand, the amount of aperture of the iris 2 is detected by an iris encoder 11, the detected amounts are amplified by a buffer amplifier 12 to a prescribed level, and an iris control signal is corrected by the amplifiers 13 and 14. The loop gain of the feedback loop of an iris control system is arbitrarily corrected by the encoder 11, amplifier 12, and amplifiers 13 and 14.