A signal processing apparatus 30 includes a FIR filter 33 that generates a signal corresponding to an instrument sound including a resonance of a body caused by a vibration of a string, based upon a pickup signal from a pickup sensor 21. The signal processing apparatus 30 guides the pickup signal from the pickup sensor 21 to an adder circuit 35 via a low-pass filter 32, guides the signal generated by the FIR filter 33 to the adder circuit 35 via a high-pass filter, and adds and mixes these signals in the adder circuit 35. The cut-off frequency of the high-pass filter 34 is equal to or higher than the cut-off frequency of the low-pass filter 32.