To provide a method and device for detecting welding zone defect, capable of detecting not only the surface but also the internal part or the depth part of the welding zone, and defect of a base material near the welding zone, without applying preprocessing to the welding zone.

An alternating current magnetic field is applied from an exciting coil from an exciting coil to the welding zone where the defect is to be detected, induced electromotive force is generated in an induction coil by the alternating current magnetic field passing the welding zone, and the variation amount of at least one of the amplitude and phase of the induced electromotive force occurring in the induction coil with respect to at least one of the amplitude and phase of a reference induced electromotive force is determined. Thus, the defect in the welding zone is detected.