A magnetic field detecting circuit has a relaxation oscillator SQUID. An oscillator circuit includes a coil and a resistor connected together in series. The oscillator circuit produces an output signal having an oscillating frequency dependent on an applied DC bias current. A magnetic field input circuit is magnetically coupled with the coil of the oscillator circuit for applying a magnetic field. A Josephson device is connected in parallel with the oscillator circuit for converting the magnetic field to an electric signal for varying the oscillating frequency dependent on the magnetic field. The magnetic field is detected by detecting a variation of the oscillating frequency. The sensitivity of the magnetic field detecting circuit is improved by magnetically coupling an RF-SQUID with an input coil to form the magnetic field input circuit. To provide a multi-channel magnetic field detecting circuit, a plurality of oscillator circuits are connected in series with each other, and a respective Josephson device is connected in parallel with each corresponding oscillator circuit. A plurality of magnetic field input circuits are each magnetically coupled with the coil of a respective oscillator circuit, and a plurality of control lines are magnetically coupled with each respective Josephson device to vary the critical current of the Josephson device to thereby control the oscillation frequency of each oscillator circuit.