To provide a pulse width modulation amplifier capable of being constructed at a low cost and of providing a high quality output by reducing noise through just digital signal processing not without feedback processing nor analog processing. An envelope detection circuit 3 detects an envelope of inputted digital data. A correction circuit 2 performs distortion correction, f characteristic (frequency characteristic) correction, and [Delta] [Sigma] correction on the input digital data. A level detection circuit 4 detects the level of the input digital data from the output of the detection circuit 3. A PWM carrier creation circuit 7 creates a carrier signal with a frequency corresponding to the output result of the level detection circuit 4. An interpolation calculation circuit 8 generates interpolation data synchronous with the carrier signal from the input digital data. A PWM circuit 10 converts the interpolation data into a pulse width modulation signal based on the carrier signal.