Image sensor and method of controlling image sensor

An image sensor is disclosed. A first thin-film transistor includes a first gate and a second gate. The first gate is supplied with a signal generated by a photoelectric conversion element. The second gate is supplied with a potential different from a potential of a first signal line by a predetermined voltage through a second signal line. The second gate has a smaller capacitance than the first gate. A second thin-film transistor supplies a reset potential received from a reset power line to the photoelectric conversion element. Whether a third thin-film transistor is in a conductive state is controlled by a selection signal. The third thin-film transistor is disposed between the first signal line and the first thin-film transistor and the current from a constant current source flows into the first thin-film transistor via the third thin-film transistor in a conductive state.