A distortion effector includes a calculation section for performing a predetermined higher-degree expression calculation on an input digital sound signal, and a multiplication section that is provided in series with the calculation section for multiplying the digital sound signal by a coefficient that is adjusted so as to utilize a specific portion of the higher-degree expression which exhibits a smooth distortion characteristic. The digital sound signal is distorted in accordance with a smooth non-linear curve based on the higher-degree expression calculation, to thereby achieve a mild distortion effect. In a multi-effector capable of imparting three or more predetermined effects in such a manner that the effects can be selected to be imparted independently of each other, a registration device is provided for registering, for each of the effects, whether the effect should be made switchable or not. When a collective-switching switch is operated, control is performed such that the effect-on or effect-off state is inversively switched for any effect registered as switchable. For any effect registered as non-switchable by the registration device, the effect-on or effect-off state is not switched even when the collective-switching switch is operated.