A mixture ratio detecting apparatus comprises: a light source which emits light including light of an infrared region; a first light detecting unit which detects the light of the infrared region from the light source; a second light detecting unit which detects light of a region having a range different from that of the first light detecting unit; and a control unit which outputs a signal pertaining to a mixture ratio based on signals from the first light detecting unit and the second light detecting unit, and controls the light source, the first light detecting unit, and the second light detecting unit.