PURPOSE: To add natural and effective modulation effect to a musical sound by providing modulation effect speed control means which increase the speed of the modulation effect as the depth of modulation effect is increased and a modulation effect depth control means which increases the depth of the modulation effect as the speed of the modulation effect is increased.

CONSTITUTION: When the depth of the modulation effect is increased by an operating element means, the modulation effect speed control means 2, 4 and 5 increase the speed of the modulation effect according to the depth of the modulation effect. When the speed of the modulation effect is increased by the operating element means, the modulation effect depth control means 7 increases the depth of the modulation effect according to the depth of the modulation effect. Then a modulation effect adding means 9 adds the modulation effect to a supplied musical sound signal according to the depth and speed of the modulation effect. Consequently, the natural and effective modulation effect is added to the musical sound.