A monitor-light-emitting device and multiple light-emitting devices are mounted on a board, and a light-guiding member is disposed in front of these devices. Monitor light emitted from the monitor-light-emitting device is directly supplied to a light receiving device. Part of light emitted from the multiple light-emitting devices is incident on the light-guiding member and is used as reference light. The reference light is received by the light receiving device. Reflected detection light that has been reflected off a target object located in front of the optical sensor is transmitted through the light-guiding member and is received by the light receiving device. A condenser is disposed in front of the light receiving device and the reference light or the reflected detection light is efficiently supplied to the light receiving device.