A light receiving device includes a light receiving element and an operation unit. The light receiving element receives light that is partly blocked by an object to be measured, on first and second light receiving regions to output respective first and second output currents. The output unit outputs the first and second output current or a value based thereon to the outside. When the direction parallel to a light receiving surface of the light receiving element and perpendicular to the first direction is defined as a second direction, a length of the second light receiving region in the second direction is, entirely in the first direction, longer than the length of the first light receiving region in the second direction.