An image sensor includes a photoelectric converter, a source-follower transistor, and a selection transistor. The photoelectric converter generates electric charge in response to received light, and the electric charge varies a voltage of a detection node. The source-follower transistor is coupled between the detection node and an output node and has a first threshold voltage. The selection transistor is coupled between the source-follower transistor and a voltage node with a power supply voltage or a boosted voltage applied thereon, and has a second threshold voltage with a magnitude that is less than a magnitude of the first threshold voltage such that the source-follower transistor operates in saturation.