There is a problem that in an image sensor including an amplifier in each pixel, when a thin-film semiconductor is used as a transistor constituting the amplifier, voltage continues to be applied between source and gate of the transistor and thereby a threshold voltage value of the transistor varies, resulting in a variation of signal voltage. To solve the problem, a thin-film transistor formed with an oxide semiconductor is used as the transistor constituting the amplifier, and during a period other than a period of outputting an output of the amplifier, source potential of the transistor is controlled to be equal to drain potential thereof.