PURPOSE: To provide a new sound volume varying mechanism of a piano by which the sound volume can be changed while maintaining a touch feeling of an ordinary keyboard.

CONSTITUTION: A small sound plate 11 for a long bridge has the area smaller than the area surrounded by the outer periphery of a piano, and a long bridge 21 is fixedly arranged on the obverse side. A sound board 31 has the area almost coincident with the area surrounded by the outer periphery of the piano, and a metallic member 41 for a long bridge is fixedly arranged on the obverse side. An operation mechanism 51 is composed of four solenoid driving cylinders 52 in total, a driving circuit 53 and a switch 54. When the switch 54 is turned off, the sound plate 31 is put in a connecting condition by contacting with the small sound plate 11 for a long bridge through the metallic member 41 for a long bridge, and when the switch 54 is turned on, the sound board 31 is put a disconnected condition by separating from the small sound board 11 for a long bridge.