Keys 1, 2 are supported by a frame 3 in a freely swingable manner. Massive bodies 8, a lower limit stopper 10 and an upper limit stopper 11 are situated below keys 1, 2. Massive bodies 8 are supported by a frame 3 in a freely swingable manner and pivot in synchronization with key-depression. A lower limit stopper 10 restricts a lower limit of pivoting range of the massive bodies 8. An upper limit stopper 10 restricts an upper limit of pivoting range of the massive bodies 8. Fixing portions (erected potions) 3g1 for fixing the upper limit stopper 11 are provided on the frame 3 in one-to-one corresponding with keys 1, 2. The upper limit stopper 11 is fixed to the undersurface of the fixing portions 3g. An upper part of each fixing portion 3g is situated inside right and left side walls of each key 1, 2.