PURPOSE: To prevent a transmitting film from choking due to filtering of the fluid including a large quantity of coal dust and much moisture by providing the transmitting film at the part where carbon monoxide gas outside a receptacle is introduced.

CONSTITUTION: A transmitting film 8 is provided at the part where carbon monoxide gas is introduced of carbon monoxide gas concentration measuring apparatus 5 which measures carbon monoxide gas concentration with the carbon monoxide gas introduced into a receptacle 7 having a measuring instrument therein. The carbon monoxide in the fluid is introduced into the receptacle 7, passing through the transmitting film 8 and is measured by a detecting part 10 to detect the carbon monoxide concentration in the fluid. In the present invention, the film is free from choking because the concentration of the carbon monoxide passed through the transmitting film is directly measured by a transmitting action without filtering the fluid including coal dust and water content or without providing a moisture removal apparatus as in a conventional apparatus.