A fall board stopper is provided for a keyboard instrument such as a grand piano, which is equipped with a fall board that is opened or closed on keys arranged on a keybed. It is basically constituted by a first stopper member and a second stopper member, which are normally separated from each other and which are respectively attached to a prescribed part (e.g., an interior wall of a side board) of the keyboard instrument and a backend portion of the fall board at a prescribed position. The first stopper member has an engagement portion that is engaged with the second stopper member when the fall board is opened, so that the fall board can be reliably stopped in position when opened upon a mechanical engagement established between the first and second stopper members.