A supply voltage controller 11 includes a simplified envelope creating unit 111 that calculates an envelope of an input audio signal according to an input audio signal from an external device, creates a supply voltage control signal so that a waveform of the supply voltage control signal follows a waveform of the envelope, and outputs the supply voltage control signal to a voltage variable power supply 12 a constant time before the input audio signal is amplified by the amplifier 14. Accordingly, an amplifying device can be provided which can reduce the distortion of the output signal not depending on a change in the slew rate of the voltage variable power supply 12 and suppress the deterioration of power efficiency.