PURPOSE: To improve an efficiency regarding the second and the first frequencies by providing a capacitor for constituting a series resonance circuit against the second frequency f2, a f2 trap circuit and a coil for constituting a series resonance circuit against the first frequency.

CONSTITUTION: A series resonance circuit having a resonance frequency f2 is formed by a capacitor C4 and a coil L1 in the first frequency f1 trap circuit A. Also, a trap circuit B is constituted so as to become a trap circuit against the second frequency f2. Therefore, it is prevented that a high frequency wave of the frequency f2 sent to a sample coil Ls from a terminal CN2 leaks toward a terminal CN1, and an efficiency regarding the f2 is improved. Also, a series resonance circuit of a resonance frequency f1 is formed by a coil L3 and a capacitor C5. Therefore, a high frequency wave of the f1 supplied from the CN1 is sent to the coil Ls without a loss. In this way, the efficiency regarding the f2 and the f1 can be improved.