PURPOSE: To detect the down-edge of a current flowing to the primary winding of a transformer by varying the state of a resistance coupling type OR circuit constituted of a resistance and a Josephson device when said current is reduced to zero.

CONSTITUTION: When the current of an input signal flowing through the primary winding of the transformer 101 increases from zero toward a direction as indicated with an arrow 121, the Josephson device 103 in the detecting circuit of a vibration-proof type Josephson down-edge is switched to a resistance state at the time point when a branch current flowing through the Josephson device 103 in the secondary current induced in the direction as indicated with an arrow 122 in the secondary winding reaches a threshold level, and the current of the secondary winding of the transformer 101 is attenuated. Subsequently, said device is switched to a resistance zero state when the input signal current is reduced to zero from a prescribed value or the maximum value, its state is held and the resistance coupling type OR circuit 102 is switched. Also, a resistance 104 operates as a damping resistance in case the Josephson device 103 is switched to the resistance state and attenuates the current of the secondary winding, and operates to eliminate the vibration at current attenuation, the attenuation time is shortened, and the circuit is operated at a high speed.