Key information designating a key for a tune to be composed and chord information designating chords for respective chord spans to constitute a chord progression for the tune are inputted to a melody generating apparatus. In the chord span to which a diatonic chord such as C major chord is designated, melody notes are generated using the scale notes of the inputted key, while in the chord span to which a nondiatonic chord such as DbM7 chord is designated, an available note scale is determined based on the key information and the chord information, and then melody notes are generated using the notes such as Ab Bb of the available note scale. As the notes which are neither the scale notes of the key nor the chord tones may be employed for a melody, the generated melody will have musical naturalness and fits the nondiatonic chords also.