An upright keyboard musical instrument has an open/close board attached to the top position of a case, wherein the lower surface of the open/close board is subjected to mirror surface finishing and is supported by a lid prop, which is rotatably arranged in proximity to the top board to rotate about a support axis and is formed in an L-shape to have a plurality of support surfaces, realizing different angles for the open/close board being opened. The mirror surface of the open/close board projects images representing internal members of the case, such as an emblem and actions. By appropriately rotating the lid prop, it is possible to realize a desired angle for the open/close board, thus realizing tone volume control and visually entertaining effects in musical performance.