The present invention relates to a detecting device for detecting an SSR signal having a characteristic structure. The detecting device comprises filtering means matched to the characteristic structure of the SSR signal, and means for maintaining a false-alarm rate at a substantially constant value. The characteristic structure of the SSR signal comprises either a preamble or an initial pulse and a final pulse separated by a fixed dwell time. The means for maintaining a false-alarm rate at a substantially constant value comprise computing means configured to compute a detection threshold on the basis of a signal supplied by the filtering means, and decision means configured to detect the SSR signal on the basis of the detection threshold and of the signal supplied by the filtering means.