An object is to provide a photosensor utilizing an oxide semiconductor in which a refreshing operation is unnecessary, a semiconductor device provided with the photosensor, and a light measurement method utilizing the photosensor. It is found that a constant gate current can be obtained by applying a gate voltage in a pulsed manner to a transistor including a channel formed using an oxide semiconductor, and this is applied to a photosensor. Since a refreshing operation of the photosensor is unnecessary, it is possible to measure the illuminance of light with small power consumption through a high-speed and easy measurement procedure. A transistor utilizing an oxide semiconductor having a relatively high mobility, a small S value, and a small off-state current can form a photosensor; therefore, a multifunction semiconductor device can be obtained through a small number of steps.