The invention relates to a device (20) for indicating the operation of an assisted breathing apparatus (5, 1) intended for a patient (3), the apparatus comprising at least one flow channel (7) for respiratory gases, said indicator device (20) comprising an inner space (27) communicating with the flow channel (7), the volume of the inner space (27) varying depending on the pressure of the gases inside the flow channel (7) relative to the pressure of the environment in which the indicator device (20) is placed. According to the invention, the device (20) comprises a rigid portion (21, 2) for coupling to the breathing apparatus (5, 1) and the flow channel (7) thereof, as well as a sealed flexible wall (22, 31) defining the inner space (27) relative to the environment, the sealed flexible wall being secured in communication with the rigid coupling portion, the rigid coupling portion allowing communication between the flow channel (7) and the inner space (27), and, with equal pressure between the environment and the inner space, at least one portion of the flexible wall has folds (25, 26) and said folds disappear or are reversed when the pressure in the inner space increases relative to the pressure in the environment due to the extension of said flexible wall (22, 31), the volume of the inner space (27), with equal pressure between the environment and the inner space, being less than 50 cm3.Linvention concerne un dispositif indicateur (20) de fonctionnement pour un appareil dassistance respiratoire (5, 1) destiné à un patient (3), lappareil comportant au moins un canal de circulation (7) de gaz respiratoires, ledit dispositif indicateur (20) comportant un espace interne (27) en communication avec le canal de circulation (7), lespace interne (27) voyant son volume varier en fonction de la pression des gaz dans le canal de circulation (7) par rapport à la pression de lenvironnement dans lequel est placé le dispositif indicateur (20). Selon linvention, le dispositif (20) c
