Abstract: An apparatus for detecting a magnetic field includes a magnetic core, an excitation current source unit, a magnetic flux detecting unit, and a current control source unit. The excitation current source unit includes a first excitation winding disposed around the core, an amplifier having an output coupled to the first excitation winding, and a subtractor for applying to the amplifier a first difference between a voltage corresponding to a supplied excitation current and a voltage corresponding to a current passing through the first excitation winding. The magnetic flux detecting unit includes a second excitation winding disposed around the core, a waveform shaping circuit for shaping the waveform of a voltage induced in the second excitation winding, a pulse generating circuit for outputting pulses in synchronism with the shape waveform voltage, and a sample-and-hold circuit for sampling and holding the voltage corresponding to the supplied excitation current.