A device for measuring, on an aircraft, the extinguishing agent concentration of a gaseous mixture. The device includes an ultraviolet radiation detector for generating an output signal that is a function of the concentration of the agent in the gaseous mixture and an electronic circuit for determining the concentration as a function of said output signal. The device includes a unit for sampling the gaseous mixture in at least one fire zone of said aircraft, and a measurement chamber passed through by the gaseous mixture and comprising a tranquilizing chamber, the detector being arranged in such a way as to measure the concentration of the agent in the gaseous mixture passing through the tranquilizing chamber.