In an embodiment of this invention, if a state, in which duty ratio of a PWM signal is kept equal to or greater than a predetermined value, has been maintained for a first time period, the duty ratio of the PWM signal of a digital power amplifier is adjusted downward. Furthermore, the duty ratio of the PWM signal is maintained without change when the duty ratio of the PWM signal is adjusted downward for a second time period, and the duty ratio of the PWM signal is adjusted downward only when volume is set to a maximum value allowable in the power amplifier. Accordingly, the temperature of the power amplifier can be prevented from rapidly increasing, the internal temperature of an electronic apparatus can be efficiently prevented from increasing, and damage to the power amplifier resulting from increased temperature can be prevented.