A device for measuring the concentration of a gas contained in a cavity and for checking the operation of a catalytic element in an exhaust line in an automobile vehicle. A first emitter (E1) composed of an optical pumped micro-cavity and for which the emission spectrum is within the gas absorption band emits a first radiation that passes through the cavity. A second emitter emits a second radiation that passes through the cavity. A receptor measures the optical intensity (I) of the radiation that passed through the cavity.