The authors examined the parametric decay instability of the linearly unstable electrostatic ion cyclotron wave in an ion beam-plasma system which decays to an ion acoustic wave and a linearly stable electrostatic ion cyclotron wave. They have derived a set of equations which represent this mechanism, and obtained numerical results of the temporal evolution of the amplitude of the related wave and distortions of the velocity distribution function. The beam distortion begins to occur at the stage of linear instability and becomes large at the stage of nonlinear instability. Ion heating of the plasma occurs only at the stage of nonlinear instability. These large distortions of the ion beam and ion heating of the plasma are caused by the deceleration of the ion beam.