A pixel circuit comprising: a photoelectric conversion element; an amplifier circuit formed of an amplifier transistor; a transfer transistor for transferring electric charge generated by the photoelectric conversion element to an input node of the amplifier circuit, wherein the transfer transistor has a first field effect transistor, a second field effect transistor, and a third field effect transistor integrated and connected in series from the photoelectric conversion element to a side of the amplifier circuit, the first and second field effect transistors (1121, 1122) have gate electrodes to be simultaneously collectively driven, and a threshold voltage of the first field effect transistor is set to be higher than that of the second field effect transistor, and as the gate electrodes are driven step by step, the electric charge generated by the photoelectric conversion element and transferred via the first field effect transistor is accumulated in a channel region of the second field effect transistor, and the electric charge accumulated in the channel region is transferred to an input of the amplifier circuit via the third field effect transistor, and wherein the amplifier circuit drives a signal line, so that the accumulated electric charge is read, characterized in that the signal line is connected between a constant current circuit and a sensing circuit, a gate electrode of the third field effect transistor has a fixed electric potential, so that the third field effect transistor functions as a separation transistor between the second field effect transistor and the input node; the electric potential of a channel region of the third field effect transistor is higher than an electric potential of the channel region of the second field effect transistor, if the second field effect transistor is in an OFF-state, and the electric potential of the channel region of the third field effect transistor is lower than the electric potential of the channel region of the second field effect transistor, if the second field effect transistor is in an ON-state. a photoelectric conversion element; an amplifier circuit formed of an amplifier transistor; a transfer transistor for transferring electric charge generated by the photoelectric conversion element to an input node of the amplifier circuit, wherein the transfer transistor has a first field effect transistor, a second field effect transistor, and a third field effect transistor integrated and connected in series from the photoelectric conversion element to a side of the amplifier circuit, the first and second field effect transistors (1121, 1122) have gate electrodes to be simultaneously collectively driven, and a threshold voltage of the first field effect transistor is set to be higher than that of the second field effect transistor, and as the gate electrodes are driven step by step, the electric charge generated by the photoelectric conversion element and transferred via the first field effect transistor is accumulated in a channel region of the second field effect transistor, and the electric charge accumulated in the channel region is transferred to an input of the amplifier circuit via the third field effect transistor, and wherein the amplifier circuit drives a signal line, so that the accumulated electric charge is read, characterized in that the signal line is connected between a constant current circuit and a sensing circuit, a gate electrode of the third field effect transistor has a fixed electric potential, so that the third field effect transistor functions as a separation transistor between the second field effect transistor and the input node; the electric potential of a channel region of the third field effect transistor is higher than an electric potential of the channel region of the second field effect transistor, if the second field effect transistor is in an OFF-state, and the electric potential of the channel region of the third field effect transistor is lower than the electric potential of the channel region of the second field effect transistor, if the second field effect transistor is in an ON-state.