An object of the invention is to improve the accuracy of light detection in a photosensor, and to increase the light-receiving area of the photosensor. The photosensor includes: a light-receiving element which converts light into an electric signal; a first transistor which transfers the electric signal; and a second transistor which amplifies the electric signal. The light-receiving element includes a silicon semiconductor, and the first transistor includes an oxide semiconductor. The light-receiving element is a lateral-junction photodiode, and an n-region or a p-region included in the light-receiving element overlaps with the first transistor.