To provide a drive circuit for a magnetic impedance effect element capable of stably detecting an external magnetic field even if there are variations in the inductance of a negatively-fedback magnetic field generation coil when negative feedback is applied to the coil to linearize a detection output characteristic in the case of a magnetic field detector for detecting an external magnetic field by using a magnetic impedance effect element.

In this circuit for detecting an external magnetic field Hex by using the magnetic impedance effect element 4 whose impedance is changed by the external magnetic field imposed thereon, an impedance correction circuit 8 is inserted between the negatively-fedback magnetic field generation coil 7 for imposing a negatively-fedback magnetic field on the element 4 and an external magnetic field detection output end 61 so that the rate of negative feedback is not affected by a change in the inductance of the coil 7.