An amplitude component and a phase component of a modulated signal are inputted into a power supply terminal and an input terminal of an RF power amplifier, respectively, and a modulated wave that is produced by demodulating an original modulated signal is obtained from the RF power amplifier. A supply voltage is supplied to an emitter follower and an operational amplifier via a switch group from a DC-DC converter group whose output voltages are sequentially different. As the supply voltage, any pair of outputs of DC-DC converters by the switch group is selected according to levels of the amplitude component, and provides them to the emitter follower and the operational amplifier. The emitter follower performs a DC conversion of the provided supply voltage to provide it to the RF power amplifier.