An image pickup apparatus includes a detector having pixels divided into first and second pixel groups, a signal processor including a reading circuit unit including first and second reading circuits, an A/D conversion unit including first and second A/D converters, a digital data processing circuit, and a controller. The controller instructs the signal processor to perform a signal processing operation several times of adding direct current potentials to analog electric signals of pixels from the reading circuit unit, supplying the signals to the A/D conversion unit, and outputting digital data to the digital data processing circuit while the direct current potentials are changed, and an average processing operation of averaging the digital data for the pixels using the digital data processing circuit.