When a sine wave is supplied from a sine wave oscillator through a resonant capacitor selection circuit, an alternating electric and magnetic field is generated on a loop coil. A magnetic intensity detection coil supplies an electromagnetic field signal to a control circuit when detecting electromagnetic intensity of the loop coil. The control circuit controls the resonant capacitor selection circuit based on the electromagnetic field signal and adjusts a maximum current to flow into the loop coil. Namely, a resonant capacitor with a suitable capacitance is selected in such a way that a resonance point (for example, 125 kHz) reaches a maximum level. Moreover, the control circuit adjusts a variable resistor to obtain a target electromagnetic field signal (intensity of electromagnetic field) in this state. Namely, automatic gain control is performed to obtain optimal electromagnetic field intensity on the loop coil.