In an A/D converter having a single flux quantum circuit having a flux quantum as an information carrier, a superconducting amplifier circuit driven by an AC current, and a semiconductor circuit, the operations of the circuits are synchronized with each other and a data signal from the single flux quantum circuit is transmitted to the semiconductor circuit.An AC current as the power source of a superconducting amplifier circuit is inputted as a master clock signal to the single flux quantum circuit and the semiconductor circuit to synchronize the operations of the circuits with the master clock signal. The single flux quantum circuit has a clock signal frequency multiplier circuit, a demultiplexing circuit and a memory circuit.