An image sensor including a pixel array having vertical signal lines, each interconnected to one of columns of the pixel array, and a column processor including a unit readout circuit provided for each of sets of a predetermined number of columns. The unit readout circuit includes input switches, each connected to a corresponding one of the vertical signal lines and being sequentially turned on and off, an input capacitor having one end commonly connected to the input switches, a reference switch for selectively providing a reference voltage to the input capacitor, an operational amplifier connected to the other end of the input capacitor, a reset switch for selectively providing a short-circuit between input and output ends of the operational amplifier, and a feedback circuit provided for each of the columns and including a feedback switch and a feedback capacitor connected in series between the two ends of the operational amplifier.