Specific pitch of an input tone signal is sequentially detected, and a normalized pitch corresponding to a pitch name is sequentially detected on the basis of the specific pitch. It is determined whether there has been a variation in the detected pitch. Lead tone (first tone signal) is generated on the basis of the input tone signal, and a harmony tone (second tone signal) is generated on the basis of the detected pitch. When it is determined that there has been a variation in the pitch, processing waits until a predetermined time passes, and control is performed to change a pitch of the second tone signal if a pitch detected immediately before the variation and a current detected pitch are determined to be different from each other upon the passage of the predetermined time.