Abstract: A muting device for a piano includes a capstan screw, a support rail, a support, a jack, a shank rail, a hammer shank, a hammer and at least one string with respect to each key. Each key is divided into a back portion and a front portion with respect to the rotation axis. This front portion of the key is depressed by a performer, while the back portion of the key is inserted into a key driving mechanism of the grand piano. In addition, a transmitting member (e.g., another capstan screw) transmits force from the key to the hammer. The transmitting member is inserted between an upper surface of the key and a lower surface of the support. The transmitting member contacts the upper surface of the key between the capstan screw and a rotation center of the key, and it also contacts with the lower surface of the support between a position at which the capstan screw comes contacts the support and a position at which the jack is connected to the support.