For protecting a circuit (1) against a mechanical or electromagnetic attack, an active protection device attached to the circuit comprises: —at least one generator (13, 14) for generating a magnetic field, —at least one magnetic sensor S1, S2, S3, S4 for measuring a value of the magnetic field, —an integrity circuit connected to the at least one magnetic sensor S1, S2, S3, S4 and to the circuit (1). The integrity circuit activates a reaction procedure in the circuit if the measured value of the magnetic field made by the magnetic sensor is out of a values domain, the values domain being correlated to the generated magnetic field.