A first key unit (37) of a first keyboard unit (3) is provided so as to be swingable on a first base plate (5), and a second key unit (47) of a second keyboard unit (4) is provided so as to be swingable on a second base plate (8). A gear teeth portion (43) formed on a circular arc face (42) of a first gear member (44) provided on one side of a first support plate (6) and a gear teeth portion (53) formed on a circular arc face (52) of a second gear member (54) provided on one side of a second support plate (9) mesh with each other.
