Abstract: An image sensor includes a plurality of photoelectric elements arranged in the form of an array and a like plurality of blocking diodes, each of which is connected in series with and opposite in direction to the corresponding one of said photoelectric elements thereby forming a charge accumulating unit. The image sensor also includes means for setting long and short charge accumulating time periods, whereby when the long charge accumulating time period is set, a combined signal including a true signal and the dark current signal is outputted and when the short charge accumulating time period is set, only the dark current signal is outputted.