It is pointed out that by placing a Josephson tunnel junction in a cavity, which resonates with the supercurrent of the junction, i.e., when the Josephson frequency ω=2eV0/h is equal to one of the cavity frequencies ωr, and the cavity Q-value is sufficiently high, then the oscillating electric-magnetic field excited by the junction reacts on the junction itself and produces a supercurrent oscillation with the applied external flux. Its period can be much smaller than the magnetic flux quantum period φ0.