PURPOSE: To easily obtain a fine sound volume change due to breathing by detecting a change in the abdomen measurement of the player based upon player's abdominal breathing by a belt put on the abdomen part and controlling the sound volume in accordance with the change.

CONSTITUTION: When the abdomen measurement is changed by abdominal breathing, the belt 1 is extended/shortened like the shown arrow 11 by the elastic force of a spring 6, the sliding chip 10 of a variable resistor 7 is slided as shown by an arrow 12 and a resistance value is changed. A signal corresponding to the change in the resistance value is inputted to a musical tone control circuit 21 in an instrument body through a cord 18 to control the sound volume. Thus, an electric signal corresponding to a change in the abdomen measurement of the player in playing is inputted from a controller 3 to the electronic instrument and sound volume control corresponding to the change in the abdoment measurement is attained. Consequently, a fine sound volume change can simply be obtained.