In a harmonic adding device, an input/output function part converts a digital sound signal having an input signal level into a digital sound signal having an output signal level determined according to a input/output function and the input signal level. A parameter storage unit stores at least one inflection point. A controller sets the inflection point to the input/output function to enable the input/output function part to add harmonics to the digital sound signal. The input/output function is defined between a positive maximum point and a negative maximum point. The inflection point is set between the positive maximum point and the negative maximum point. The positive maximum point, the inflection point and the negative maximum point are sequentially connected by linear interpolation to formulate the input/output function.