A top board structure is adapted to a keyboard instrument (e.g., a grand piano) having a pair of a top board rear and a top board front, which are arranged close to each other with a gap therebetween above the upper portion of a case. Herein, the gap is enlarged using sliders along which the top board front can be moved forwards and backwards as necessary, wherein the top board rear is allowed to be opened only when the top board front is positioned to enlarge the gap. Therefore, the user can easily hold the free end of the top board rear to be opened or closed with fingers. Thus, it is possible to prevent the player from being anxious that finger tips will be caught in the gap during an opening motion and/or a closing motion of the top board rear.