A SQUID for detecting a weak magnetic field is constructed of a sensor coil (104) which detects a signal magnetic flux, and a superconducting loop (103) which is magnetically coupled with the sensor coil (104) to generate a periodic voltage corresponding to an interlinking magnetic flux from the sensor coil (104). The periodic voltage from the SQUID is amplified, and is taken out. The periodic voltage taken out is synchronously detected by a phase-sensitive detecting amplifier (109). The synchronously-detected signal is converted into a light signal, which is fed back to the SQUID (103, 104). The light signal fed back is converted into an electric signal by a superconducting phototransistor (106) which is arranged in a cryogenic atmosphere (101). A magnetic flux corresponding to the electric signal from the superconducting phototransistor (106) is fed back to the superconducting loop (103) by a feedback coil.