PURPOSE: To simplify constitution by providing a work coil grounded in its center tap on the output side of a matching device to sum power in the work coil.

CONSTITUTION: A RF signal transmitted from an oscillator 1 is amplified by a preamplifier, then divided into two by a power divider 3, and transmitted to first and second power amplifiers 4a, 4b with the phases being shifted by 180°. The RF power amplified by the power amplifiers 4a, 4b is transmitted through transmission lines 5a, 5b, subjected to impedance conversion by a matching device 6, and then sent to a work coil 7. The work coil 7 is grounded in its center so that a power is summed in this coil 7 when a waveform of reversed phase is sent from both ends of the work coil 7. Thus, the power sum and impedance conversion in the course are not required, and the constitution is simplified.