A lead tone is generated on the basis of an input tone signal. Meanwhile, a specific pitch of the input tone signal is sequentially detected, from which is detected a normalized pitch corresponding to any one of the musical pitch names. Then, difference information is obtained which pertains to a difference between the specific pitch and the normalized pitch, and a pitch having a given pitch interval from the normalized pitch is determined as a target pitch of a tone signal to be generated. Then, a harmony tone is generated which has a pitch obtained by modulating the target pitch in accordance with the difference information.