The optical sensor includes a substrate, a first transistor for functioning as a light-receiving element and a second transistor for writing/reading in a pixel region provided on the substrate. The first transistor is formed by a transistor using polycrystalline silicon, the second transistor is formed by a transistor using an oxide semiconductor. A light-shielding layer is provided on the back side of the oxide semiconductor of the second transistor. Thus, it is possible to irradiate light to the optical sensor fora long time, and in addition to increasing the amount of light received by the first transistor, it is possible to suppress variations in the characteristics of the second transistor.