A device for the detection of an unauthorised liquid or solid substance in a zone with protected access. The device includes a support base designed to receive a single foot, wearing a shoe of an individual to be checked, an electromagnetic field emitter/receptor operating at least at several frequencies in a specified range of frequencies, a position marker, on the support base, designed to impose a precise positioning of the foot wearing the shoe in relation to the emitter/receptor. The complex impedance is measured of the emitter/receptor influenced by a load which consists of the shoe and its contents, representing the complex dielectric characteristics of the shoe and of its contents. A mechanism is provided for supplying information associated with the measured complex impedance and therefore with the nature of the contents of the said shoe of the individual to be checked.