An automatic player musical instrument is a combination of an acoustic piano and an electric system, which serves as an automatic playing system and an electronic proof system, and a controller of the electric system has an information processing capability; when a user instructs the electric system to check performance on the keyboard, a subroutine program for electronic proof starts to run; while the user is fingering a music tune, the controller monitors the keys with key sensors to see whether or not the user correctly plays the music tune; when the user mistakenly depresses a key instead of another key, the controller gives rise to vibrations of the mistakenly de-pressed key so as to give a notice of incorrect fingering to the user.