To calibrate offset using an inexpensive convenient circuit by determining an offset calibration signal using an angle signal being delivered from a Wheatstone bridge in the absence of DC field as a measure of offset of the Wheatstone bridge.

Total value of output voltages from Wheatstone bridges 1, 2 is maximized only when a DC field acts on the Wheatstone bridges in a direction perpendicular to a first pair of reluctance 3 or a second pair of reluctance 4. After the DC field is removed, a larger shift of a measured output voltage from an offset voltage is measured. Consequently, the output voltage being removed by the Wheatstone bridges 1, 2 is a measure of offset voltage of the Wheatstone bridge. A voltage being delivered from the Wheatstone bridge under absence of DC field is employed by an evaluation circuit for offset correction.