Abstract: A highly sensitive magnetic field detecting system includes a DC superconducting quantum interference device (DC SQUID) for detecting a magnetic field to produce a signal voltage, and a flux locked loop (F.L.L.) circuit device for driving the DC SQUID. A phase detecting circuit forming part of the F.L.L. circuit device is composed of two phase detecting circuits having different phases to be detected. By adding the individual output signals of the two phase detecting circuits, the outputs can be unvaried at a phase difference equal to that of the detected phases of the two detecting circuits. The added signal is also unvaried with respect to the phase variations of the detected signal so that the metering procedures can be simplified without any necessity for phase adjustment. Further, the output signals obtained can be stable relative to the phase drifts of the signal to be transmitted from the DC SQUID to improve the metering precision.