In an image sensor unit including a plurality of sensor chips mounted in a line on a sensor substrate, image signals from the sensor chips are sequentially read according to a reading order among the sensor chips, a current consumption circuit that consumes set current is arranged, and the current consumption circuit starts consuming the set current in response to a signal indicating end of reading of the image signals by all of the sensor chips and ends consuming the set current in response to a signal indicating start of reading of the image signals first by the sensor chip after a start signal to thereby reduce variation in the current consumption of the image sensor unit.