A class D power amplifier (1) switches a power supply voltage V.sub.DD at high-speed in response to an input digital audio signal P.sub.in to power-amplify the digital audio signal P.sub.in, and supplies a speaker (2) with the amplified signal. In the case of being incorporated in an AM receiver formed integrally with an AM tuner (3) for receiving AM broadcasts, the class D power amplifier (1) can reduces influence of reception interference with respect to the AM tuner (3). As a result, it is possible to provide a power amplifier that can suppress unnecessary radiation to reduce reception interference with respect to the tuner, not by changing a carrier frequency, but by controlling the drive signal generation systems.