A key drive device to be installed on a musical instrument with a keyboard partly located above a leg block via a key bed, including: drive units to be at least partly inserted into a removal space in the key bed. In a state in which the drive units are installed, at least one drive unit which drives at least one key located above the leg block includes: (a) a first portion at least a part of which is inserted into an insertion space portion of the removal space between the leg block and the keyboard; and (b) a second portion at least a part of which is disposed in other space portion of the removal space, a dimension of the second portion in at least one of a vertical direction and a horizontal short-side direction being larger than that of the first portion.