A manipulator information obtaining unit obtains at least a note name corresponding to a manipulator manipulated. A storage device includes a register configured to store at least possible note names, note names in a history of manipulators manipulated, and key candidates, and a first table configured to store diatonic scale notes for each of the key candidates. A key determination unit compares the note names in a history of manipulators manipulated with the diatonic scale notes for each of the key candidates and determines whether there is a key candidate in which all of the note names in the history of the manipulators coincide with any of the diatonic scale notes of the key candidate, thereby determining a key of a musical composition played by the manipulation of the manipulators, and storing the determined key in the register.