A triangular wave generating circuit (21) generates a sweep voltage of a triangular wave, a VCO (22) generates a sweep of a frequency according to the sweep voltage, and a driving coil (4) is driven by the sweep voltage. From the current flowing through the driving coil (4), an interference component is extracted and rectified by a rectifier circuit (27). A peak hold circuit (28) holds a peak value of an interference voltage within a period corresponding to one sweep. The peak value thus held is compared by a comparator circuit (29) with a predetermined value and accordingly, a relay circuit (30) outputs a signal indicative of whether an object is present or not.