A sensor main unit 1 is axially split into three sub-segments. A light-emitting element 2 is attached to one end portion, and a light-receiving element 3 is attached to the other end portion. A respiratory flow path 4 is formed so as to penetrate through the center portion. The sensor main unit 1 is attached to a position on the face below the nostrils, and respiratory gas from the nostrils is guided into the respiratory flow path 4 and is caused to cross over the optical axis connected the light-emitting element 2 and the light-receiving element 3, thereby measuring a carbon dioxide gas in the respiratory gas.