Influence of external light is suppressed. With a photodetector including a photodetector circuit which generates a data signal in accordance with illuminance of incident light and a light unit which overlaps with the photodetector circuit, a first data signal is generated by the photodetector circuit when the light unit is in an ON state, a second data signal is formed by the photodetector circuit when the light unit is in an OFF state, and the first data signal and the second data signal are compared, so that a difference data signal that is data of a difference between the two compared data signals is generated.