Parametric amplification mechanism of sound is described by using computer simulation. Amplitude-modulated ultrasonic sound with two frequencies can generate parametrically a difference-frequency sound. The results obtained by the computer simulation are that the difference-frequency sound is given by the second harmonic of envelop function distorted nonlinearly by the finite amplitude effect of the amplitude-modulated carrier. Over the shock formation distance given by the maximum amplitude including in the carrier wave, the finite amplitude distortion of the envelop is changed by nonlinear dissipation at the shock fronts of the carrier. Therefore, it is found that undesired distortion of parametrically generated secondary wave is arised by the change of the envelop shape.