An image sensor such as the conventional CMOS image sensor, in which automatic controls including so-called automatic iris control and white balance adjustment for adjusting the sensor sensitivity, namely the charge accumulation time in each pixel, according to the brightness of the image sensing ambience are performed, involves the problem that, when the frame rate of the image sensor is slowed to save power consumption, the operation of the automatic control systems will also become slower and the image quality deteriorates. In the invented image sensor system using a CMOS image sensor, while a CMOS image sensor is operated at the full frame rate all the time, a circuit for processing image signals from the CMOS image sensor is operated at a speed close to that of full frame processing only when the power supply is turned on or when the image sensing ambience varies and switched to a lower frame processing speed when automatic controls, including iris control, have become stabilized.