PURPOSE: To detect an external magnetic field with high precision by disposing a SQUID so that the external magnetic field acts thereon directly and by detecting the external magnetic field thereby directly.

CONSTITUTION: A SQUID 2 for detecting an external magnetic field is disposed on the upper side of a cylindrical insulating base 1 in a low-temperature tank 7. Then, an RF coil 3 constructing a resonance circuit 5 together with a capacitor 4 is coupled inductively to the SQUID 2. Moreover, a feedback coil 6 is so disposed as to be coupled inductively to the SQUID 2 and output signals of an integration-type amplifier 16 are taken out as precise magnetic flux detection signals V0 from the opposite ends of a resistance R0. By using the SQUID 2 itself as a detecting coil in this way, a separate detecting coil, an input coil, etc., can be dispensed with, the effect of an offset magnetic flux to be trapped thereby is removed, and thus the magnitude of an external magnetic field Bx can be detected with high precision.