A sound effect adding device includes an instruction storage section in a DSP for storing a main routine including a program for controlling an input and an output of an audio signal, and a plurality of selected sound effect adding routines transferred from a master CPU from among selectable sound effect adding routines when necessary and operable when accessed by the main routine. The main routine has a function to assign the input audio signal to the selected sound effect adding routines and collect sound effect added signals outputted from the selected sound effect adding routines so as to provide an output signal. By changing coefficient data transferred from the master CPU and stored in the DSP, the main routine performs change in connection between the input and output signals so as to change a combination of the sound effects without altering the main routine. When changing one of the selected sound effect adding routines, the master CPU transfers only a newly selected sound effect adding routine to the instruction storage section.