A CPU 41 acquires a string vibration signal in a case where a string picking operation is performed with respect to the stretched string 22, analyzes a frequency characteristic of the acquired string vibration signal, determines whether or not the analyzed frequency characteristic satisfies a predetermined condition, and changes a frequency characteristic of a musical sound generated in the connected sound source 45 depending on a case where it is determined that the predetermined condition is satisfied or determined that the predetermined condition is not satisfied.