A selected span of a music composition data file containing note data in the MIDI format which discriminates twelve notes per octave is examined with respect to respective tonalities to find an optimum tonality. Out of all the notes contained in the selected span, the number of notes that fall on the intrinsic notes of a tonality is counted with respect to each of the tonalities. The tonality which has the greatest count of intrinsic notes is determined to the optimum tonality. When the music composition is transposed, the chord names are altered according to the amount of the transposition. Then, the altered chord names are subjected to adjustment in expression according to the musical grammar. Accidental signs are adequately determined from among enharmonic notations.