A memory 33 storing in advance data of signals supplied from a detector unit 26 when a plurality of metallic foreign matter samples differing in magnitude are caused to pass an alternating field and data representing the magnitude of each foreign matter sample is provided. Setting means 32 causes an indicator 36 to display the magnitude of metal detectable by determination means 31 on the basis of those data on the foreign matter samples. Further, contained metal displaying means 34, when the determination means 31 has determined that metallic foreign matter is contained in the object of inspection, causes the magnitude of that metal content to be displayed on the indicator 36. The user is enabled to intuitively grasp the detectable magnitude of metal and the magnitude of metal contained in the object of inspection.