A circuit for detection of the presence of a permanent magnet in the vicinity of an active implantable medical device, in particular a pacemaker, a defibrillator, a cardioverter and/or a multisite device. This circuit includes a coil (12), able to respond to the presence of the magnet (16) so as to present a decreased value of its inductance, a resonant LC circuit (10) including the aforementioned coil, a generator (18) for pulse excitation of the resonant circuit; a circuit to analyze the pulse response of the resonant circuit, and a discriminating circuit, able to evaluate the value of the coil inductance from the analyzed pulse response and, correlatively, to determine therefrom the presence or the absence of the magnet. The excitation generator is able to deliver to the resonant circuit a burst of N successive pulses, and the analyzing circuit determines N corresponding pulse responses, and the discriminating circuit determines the presence of the magnet if, among N analyzed pulse responses, a number M (M≦N) of them correspond to a decreased value of the inductance of the coil.