An image sensor includes a charge pump circuit supplying first to third signals having sequentially decreasing voltage levels, a reset transistor having a drain and a gate connected with the charge pump circuit to form a diode connection and receiving the first to third signals, a photodiode generating photocharges, a transfer transistor forming a series connection between the photodiode and the reset transistor, a floating diffusion region forming a parallel connection between the transfer transistor and the reset transistor and storing the photocharges, and a drive transistor connected with the floating diffusion region, the reset transistor, a power supply voltage terminal, and a unit pixel output terminal. A gate of the transfer transistor receives a turn-off voltage if the first or second signal is supplied, and receives a turn-off voltage or a turn-on voltage if the third signal is supplied.