In the exposure control circuit of an image input device according to the present invention, the rays of light from the subject pass through a diaphragm adjusting the amount of light and undergoes photoelectric conversion at a CCD imaging element. The image signal read from the CCD imaging element is accumulated more than once at an accumulating circuit. The luminance of the subject is detected at a luminance detector. Based on the detected luminance, a condition setting circuit sets the amount of incident light, exposure time, and the number of additions to suitable values. With those suitable settings, the amount of light through the diaphragm, the exposure time of the CCD imaging element, and the number of accumulations by the accumulating circuit are controlled so as to enable the subject to be photographed under the best condition.