This invention has for purpose to provide a photosensor that is small in size and can obtain high-contrast image data and to provide a semiconductor device including the photosensor. In the photosensor including a light-receiving element, a transistor serving as a switching element, and a charge retention node electrically connected to the light-receiving element through the transistor, the reduction in charge held in the charge retention node is suppressed by extending the fall time of the input waveform of a driving pulse supplied to the transistor to turn off the transistor.