An object is sensed without a malfunction due to disturbance light. An optical sensor (101) of an aspect of the invention includes: a light-emitting element (LED) which turns off and turns on alternately for each one period; a light-receiving element (PD1) which receives reflection light and generates a photocurrent; and an integrating circuit (11) which integrates the photocurrent in a positive direction in a period during which the light-emitting element turns on and integrates the photocurrent in a negative direction in a period during which the light-emitting element turns off. A sum of lengths of integration periods in a first period and a last period is same as a length of another integration period.