A pulse width modulated signal amplifier includes an integrator having a positive or non-inverted input terminal supplied with a modulating signal, such as, an audio signal, and an inverted input terminal supplied with a rectangular wave signal as a carrier, a high gain amplifier receiving the output of the integrator and producing a pulse width modulated signal, a pulse power amplifier receiving the pulse width modulated signal, a low pass filter receiving the output of the pulse power amplifier and producing an amplified demodulated signal corresponding to the original modulating signal and which is supplied to an output terminal, and a negative feedback circuit connected between the output terminal of the pulse power amplifier and the inverted input terminal of the integrator.