Abstract: This detector comprises a variable radio frequency oscillator (1) producing a first electrical signal, a transmission circuit (3) with a capacitive antenna to which said first signal is applied, a reception circuit (6) forming, with the transmission circuit (3), a selective coupler for picking up a second electrical signal at the frequency of the oscillator, and a circuit (9) for comparing the phases of the first and second electrical signals and producing an output signal in response to a variation of the phase shift between said signals due to the presence in the surroundings of the antenna of a foreign body to be detected.The detector further comprises a feedback circuit (13) driven by the output signal of the comparison circuit (9) and controlling the oscillator (1) in order to make the frequency of the first signal vary in the sense which tends to cancel out said variation of the phase shift.