To eliminate the need for feeding many (up to 100) single magnetic flux quanta back to an integrating circuit for higher performance of a single flux quantum Σ-Δ type A/D converting circuit having a magnetic flux integrator.

This circuit comprises a loop line where a pulse signal is circulated, a pulse generator which inputs the pulse signal to the above loop line, a modulator which modulates the propagation speed of the pulse with an input signal, a switch which measures the circulation time of the pulse signal and changes an output terminal for the pulse and is arranged on the loop line, and a delay line which gives a delay to the circulation time of the pulse signal, and is characterized by that Σ-Δ modulation is performed by regarding time variation in the cycle of pulses circulated in the loop line as an integral quantity and the delay time of pulses due to the delay line as a difference quantity.