Abstract: A system for improving a spatial effect of stereo sound or encoded sound when producing three dimensional image sound signals from signals of stereo channel includes a spatial effect enhancing portion where a signal for enhancing spatial effect and directivity of sound is produced, a band enhancing portion where a signal for enhancing a signal component of the stereo channel signal in a low frequency range and for maintaining the signal component in a middle frequency range is generated, and a matrix portion where the output signal of the spatial effect enhancing portion, the output signal of the band enhancing portion and the stereo channel signal are calculated in a matrix manner, so that the spatial effect of sound is improved using a differential component between left and right side channel signals.