The invention relates to an indicator device (20) for operating a respiratory assistance device (5, 1) for a patient (3), the apparatus comprising at least one respiratory gas circulation channel (7), said indicating device (20) having an internal space (27) in communication with the circulation channel (7), the internal space (27) having its volume varying as a function of the pressure of the gases in the circulation channel (7) by relative to the pressure of the environment in which the indicator device (20) is placed. According to the invention, the device (20) comprises, on the one hand, a rigid connection part (21, 2) to the respiratory apparatus (5, 1) and its circulation channel (7) and, on the other hand, on the one hand, a closed flexible wall (22, 31) sealed delimiting the internal space (27) with respect to the environment, the flexible sealed wall being fixed in communication on the rigid connection part, the rigid connection part allowing communication between the circulation channel (7) and the internal space (27), and equal pressures between the environment and the internal space at least a portion of the flexible wall has folds (25, 26) and said folds disappearing or reversing when the pressure in the internal space increases relative to the pressure in the environment due to the extension of said flexible wall (22, 31), the volume of the internal space (27) to equal pressures between the environment and the internal space being less than 50 cm3. L'invention concerne un dispositif indicateur (20) de fonctionnement pour un appareil d'assistance respiratoire (5, 1) destiné à un patient (3), l'appareil 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), l'espace 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 l'environnement
