An image-processing device is provided that applies gate voltage in order to obtain a high quality image. The image-processing device comprises a solid-state image pickup element provided with a plurality of unit pixels each including a photo diode and a plurality of transistors for detecting an optical signal, a circuit for changing gate-applied voltage that changes a voltage applied to each gate of a plurality of the transistors. The circuit for changing gate-applied voltage applies a predetermined voltage to each gate of a plurality of the transistors from the first voltage source while in an accumulation state when carriers are generated from the photo diode in response to received light, and/or applies another predetermined voltage from the second voltage while in a reading out state when a signal in response to carriers accumulated in the accumulation state is read out.
