PURPOSE: To increase the linearity of an output signal to an incident luminous amount without being affected by the response speed and performance of a switch circuit by constitution that an electric charge of an optical storage type image pickup element is read by timewise integration.

CONSTITUTION: The optical storage type image pickup element 1 receives an input light L and converts the optical irradiated amount into electric charge amount. Every time a clock signal of a prescribed period is set, a switch SW2 is closed, an SW9 of an integration circuit 7 is selected to the terminal 11 position and an SW12 is selected to the terminal 14 position, a current by the stored charge of the element 1 is integrated and a capacitor C1 of the circuit 7 is charged. When the signal reading is finished with the clock signal turned off, the SWs 2, 9 are opened, the SW12 is selected to the terminal 13 position and the charge in the C1 is discharged via a resistor R1. An output VO from the circuit 7 is detected as an integration signal reset at each period, allowing to increase the linearity of the output signal to the incident luminous amount.