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 increases from zero toward a direction 121, the Josephson device 103 in the detecting device of a Josephson device coupling down-edge is switched to a resistance state at the time point when the secondary current of a direction 122 induced in the secondary winding reaches a threshold level, subsequently, 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 a resistance coupling type OR circuit 102 is switched. In an OR circuit 201 whose resistance coupling is an RCJL type, when the Josephson devices 203, 204 are switched to the resistance state, subsequently, the Josephson device 202 is switched to the resistance state, and an output signal appears in output terminals 108, 109.