A magnetic flux detector comprises a core member 12 and an armature member 11, and the core member 12 generates a magnetic flux by an exciting coil 13 when the exciting coil receives an excitation current controlled by an excitation current controller 30. The magnetic flux detector further comprises a search coil 14, which is provided in a magnetic path for the magnetic flux, and a magnetic flux calculator 40, which determines the magnitude of the magnetic flux by measuring an electromotive force induced in the search coil 14 by a change in the magnetic flux. The excitation current controller 30 controls the excitation current to increase from zero to a predetermined current value during a first predetermined time period T1 and then to decrease from the predetermined current value to zero during a second predetermined time period T2, and the magnetic flux calculator 40 determines the magnitude of the magnetic flux based on the difference between the increasing electromotive force integral value which is calculated by integrating the electromotive force induced in said search coil 14 during the first predetermined time period T1 and the decreasing electromotive force integral value which is calculated by integrating the electromotive force induced in the search coil 14 during the second predetermined time period T2.