Exhaust gas discharged from an exhaust port is taken into an exhaust gas sampling unit together with the surrounding outside air. A leak detection mechanism, which detects the leakage of exhaust gas from a sampling port, includes a temperature sensor including an outside and an inside temperature sensors, and a temperature measuring instrument for measuring a temperature difference between inside and outside temperatures detected by the temperature sensor. A plurality of inside/outside paired temperature sensors is disposed in the circumferential direction of the sampling port. When a leak occurs, the temperature of the inside temperature sensors rises and the temperature difference between the inside and the outer and outside temperature sensors increases, and the temperature measuring instrument detects the leak. When a leak occurs in a portion of the circumferential direction, only the temperature difference of the portion where the leak occurs is increased, and the location of the leak is identified.