When no light enters a photodiode, a second current mirror circuit operates. Consequently, a current that decreases a potential difference between two terminals of a light-receiving element flows into a transistor of the second current mirror circuit and an output of an optical sensor becomes low. Meanwhile, when light enters the photodiode, a transistor of the light-receiving element is turned on and the second current mirror circuit stops operating. Consequently, the current that decreases the potential difference between the two terminals of the light-receiving element is stopped in the transistor of the second current mirror circuit and the output of the optical sensor becomes high. This makes it possible to provide an optical sensor that has a circuit in which an output of a light-receiving element does not depend on a photocurrent and that is capable of operating at a high speed.