In a power amplifier driven by a pulse width modulation (PWM) signal, a first pair of drive pulses opposite in level to each other is formed from a first pulse width modulation signal whose quantization level corresponds to its pulse width and supplied to a first push-pull circuit (15). A second pair of drive pulses opposite in level to each other is formed from a second pulse width modulation signal whose two's complement of quantization level corresponds to its pulse width, and supplied to a second push-pull circuit (16). A speaker (19) is connected between the first and second push-pull circuits (15 and 16). A deviation between potentials at the output terminals of the first and second push-pull circuits (15 and 16), respectively, is detected. When a deviation is detected, the push-pull circuits are substantially stopped from operating.