PURPOSE: To provide a sound effect adding device which is capable of effectively utilizing the characteristics of timber according to the kinds of the timber and ways of applying a sound effect and is capable of naturally and smoothly varying the sound effects without changing a sound volume feel before and after application of the sound effect.

CONSTITUTION: An electronic musical instrument has an input means 1 for inputting an input sound signal, an effect setting means 17 and/or timbre setting means 15 for setting the effect to be applied on the inputted input sound signal, an effect component forming means 3 for forming a sound effect component in accordance with the input sound signal, an effect sound control means 4 for controlling the output value of the formed effect component, a mixing means 5 for mixing the input sound signal and the controlled effect sound and an output means 9 for outputting the mixed sound. The instrument has a control value determining means 6 for determining the control value of the sound signal control means 7 for controlling the sound signal outputted from the mixing means 5 in accordance with the set value of the effect sound set by the effect setting means 17 and a sound signal control means 7 for controlling the sound signal outputted by the mixing means 5 in accordance with the determined control value.