PURPOSE: To provide an automatic music composing device which can compose and play music based upon sounds corresponding to optional operated keys as motif through operation of the keys on a keyboard that anybody can easily operate.

CONSTITUTION: A music signal generated by pressing plural optional keys 10a on the keyboard 10 in order is converted into a digital signal, which is outputted to a music composing process circuit 13. The music composing process circuit 13 generates music based upon music logic according to predetermined music composing algorithm on the basis of the number, the pitches, and order of the input sounds. The generated music data are outputted to an automatic performance control circuit 14, which controls a keyboard driving device 15 to automatically play the composed music.