PURPOSE: To operate a final stage amplifier with a small loss and to improve the reliability by operating the final stage amplifier at the saturation region independently of the quantity of output power.

CONSTITUTION: A high frequency oscillating signal whose frequency is, e.g. 13.56MHz is used for the oscillating signal from a crystal oscillator 3 and a transformer is used for a 1st stage amplifier 5 and a final stage amplifier 7 respectively. The final stage amplifier 7 is normally operated at a saturation region. Moreover, the efficiency is constant independently of a DC power supply voltage and the output power and the loss are both in proportion to the DC power supply voltage. Thus, the output power is limited by supply power of the final stage amplifier 7. Thus, the final stage amplifier 7 is operated at the saturation region independently of the quantity of the output power at a small loss and the reliability is improved.