The invention intends to accurately measure a concentration of an object. An optical sensor (1) includes a detection light source (10) that emits measuring light, a light detection unit (40) that detects light intensity of the measuring light, which has been changed due to a change in optical characteristics of an object detection member (20), and a reference light source (11) that emits reference light. Light intensity of the reference light is detected by the light detection unit (40) without being affected by the change in the optical characteristics of the object detection member. The two light sources are arranged under the same environment.