An automatic key adjusting apparatus is provided, which determines keys on an input melody in real time and adjusts the determined keys in non real time to obtain accurate keys, thereby enhancing accuracy of placement of chords. The automatic key adjusting apparatus is provided with a keyboard for playing a melody of a musical piece. CPU judges keys on the melody in real time based on a history of pitches of the played melody of the musical piece, and adjusts the result of the key judgment in non real time after the melody of the musical piece is played.