An apparatus and a method for detecting the structure of a music piece which produces partial music data pieces each including a predetermined number of consecutive chords starting from a position of each chord in chord progression music data; compares the partial music data pieces with the chord progression music data to calculate degrees of similarity for each of the partial music data pieces; detects a position of a chord in the chord progression music data where the calculated similarity degree indicates a peak value higher than a predetermined value for each of the partial music data pieces; and calculates the number of times that the calculated similarity degree indicates a peak value higher than the predetermined value for all the partial music data pieces for each chord position in the chord progression music data to produce a detection output representing the structure of the music piece.