There is provided an optical sensor in which a reduction in size of a light-receiving part is achieved. This optical sensor is installed in a device, and includes a light-receiving element 4 for receiving light coming from outside toward the device. The optical sensor detects the state of the received light. A linear optical waveguide 2 is connected to the light-receiving element 4 so as to be capable of light propagation. The optical waveguide 2 has a front end portion serving as a light entrance portion on which light coming from the outside of the device is incident.