Provided is a proximity sensor using a photosensor, which is easy to use and reduced in power consumption. In the proximity sensor, a first photosensor is used to detect a change in amount of ambient light entering the first photosensor, which is caused when a finger is coming close thereto, and a detection signal is output based on a result of the detection. The photosensor includes, for example, one or a plurality of PN junction elements connected in parallel.