A keyboard musical instrument such as a grand piano has a front top board and a rear top board with which an upper opening of a piano case is closed, and the rear top board is confronted with the front top board at the closing position; an automatic board spacer is provided between the front top board and the piano case, and automatically increases the gap after the rear top board leaves the closing position and decreases the gap at the arrival of the rear top board at the closing position, thereby preventing the user's fingers from pinch between the front and rear top boards.