[Problem] To inhibit the waveform distortion of pseudo-sine waves. [Solution] In a stimulus signal, a positive pulse group composed of a plurality of pulses with potential rising to the plus side and a negative pulse group composed of a plurality of pulses with potential falling to the minus side appear alternately at a predetermined cycle. When one cycle of the stimulus signal is divided into four periods consisting of a first period, a second period, a third period, and a fourth period, the positive pulse group appears in the first period and the second period, and in the first period the pulse width gradually increases, while in the second period the pulse width gradually decreases so as to exhibit time changes that are the inverse of the time changes in the pulse width in the first period. The negative pulse group appears in the third period and the fourth period, and in the third period the pulse width gradually increases, while in the fourth period the pulse width gradually decreases s so as to exhibit time changes that are the inverse of the time changes in the pulse width in the third period. This stimulus signal is characterized by having a discharge pulse with potential rising to the plus side immediately before the positive pulse group, and a discharge pulse with potential falling to the minus side immediately before the negative pulse group.Le problème décrit par la présente invention est d'empêcher la distorsion de forme d'onde d'ondes pseudo-sinusoïdales. À cet effet, dans un signal de stimulus, un groupe d'impulsions positives composé d'une pluralité d'impulsions ayant un potentiel croissant vers le côté plus et un groupe d'impulsions négatives composé d'une pluralité d'impulsions ayant un potentiel descendant vers le côté moins apparaissent en alternance à un cycle prédéterminé. Lorsqu'un cycle du signal de stimulus est divisé en quatre périodes consistant en une première période, une deuxième période, une troisième période et une quatrième période,
