A CPU 41 determines whether or not the detected level of string picking strength exceeds a predetermined first level, and in a case of determining that the predetermined first level is exceeded, determines whether or not a condition is satisfied that the number of the frets 23 in contact with the string 22 detected as a picked string is a predetermined number or more (10 or more) while the frets in contact therewith as above are located within a predetermined area from the bridge 16 (the fret number 18 or higher). In a case where it is determined that the condition is satisfied, the CPU 41 instructs the connected sound source 45 to generate a predefined slap sound.