The invention concerns a device for monitoring an imaging or radiotherapy unit (1) for treating members of the human body subject to displacements related to movements of the diaphragm. The invention is characterized in that said monitoring device is designed to enable, in a prior preparation phase, to store two values, called rest value and triggering value, respectively representing, for each patient, of his suspended ventilatory level and an inhalation or exhalation ventilatory level, triggering acquisition of images or irradiation, then, during the real monitoring of the unit (1), in commanding an image or irradiation acquisition, once the correspondence between the measured value of the suspended ventilatory level and the stored rest value has been established, and only when the correspondence between the measured respiratory value of the patient and the stored triggering value has been subsequently established. Additionally, said monitoring device comprises means (11) for displaying the respiratory curve of the patient, which can be viewed by him, and whereon are physically represented the rest and triggering values.