PURPOSE: To play performance of scores of full scales by driving fingers of a robot to hit a narrow range of a piano having a small number of keys, and performing an octave-transposition converting process in a computer by a preliminarily formed program.

CONSTITUTION: An electronic piano 1 is provided with a small number of 12 keys 2 for an octave at least. A robot 3 is provided with fingers 6, 7 on the keys 2, drive parts 10, 11 to hit the keys 2 by the fingers 6, 7, and drive parts 8. 9 to move the fingers 6, 7 in the range of the key 2. In addition, a performance information processing command system by a program provided with a computer 12 to operate the robot 3 is provided. A performance program is formed preliminarily, performance information is rewritten from an original score to be inputted into the computer 12, the drive parts 8-11 are actuated by output of signals to an operation system of the robot 3 for hitting the keys 2 by the fingers 6, 7, and octave-transposition coversion is performed by the performance program for conversion tinto full scales of the original score for sound outputting.