A chord estimation apparatus includes: frequency-component extraction
means for extracting a frequency component from an input music signal;
scale-component information generation means for mapping the frequency
component extracted by the frequency-component extraction means onto each
tone and generating scale-component information including each tone and
loudness thereof; folding means for folding the scale-component
information generated by the scale-component information generation means
for each two octaves to generate scale-component information including 24
tones; and chord estimation means for inputting the scale-component
information including 24 tones into a Bayesian network in order to
estimate a chord.