An optical sensor includes: a driving circuit that turns off a light-emitting element during a first period, a second period, and a fourth period and that turns on the light-emitting element during a third period; an integrating circuit that outputs a first integrated-value difference (FID) and a second integrated-value difference (SID), the FID being a difference between an integrated value of a photocurrent generated by a light-receiving element in accordance with respective states of the light-emitting element during the first period and the second period, the SID being a difference between an integrated value of a photocurrent generated in accordance with respective state of the light-emitting element during the third period and the fourth period; and an output control circuit that outputs the SID when the FID is zero and that outputs a difference between the SID and the FID when the FID is not zero.