PURPOSE: To easily obtain a prescribed amplification factor by providing a coupling circuit including a capacitance between a Josephson junction element of an input circuit and a Josephson junction element of an output circuit.

CONSTITUTION: The voltage generated in the josephson junction element 1 of the input circuit is impressed as it is to the Josephson junction element 2 of the output circuit via a capacitor 3 of the coupling circuit to make a current over a critical current or over to flow. Thus, each Josephson junction element 2 of the output circuit is switched simultaneously to output a voltage. Thus, the output voltages are supervised to obtain a prescribed amplified voltage.