A lid opening/closing mechanism for a keyboard musical instrument, including: a main body including a casing and a keyboard; a lid attached to the casing so as to be movable between a closed position for covering the keyboard and an open position for uncovering the keyboard; and a connecting mechanism for connecting a side wall portion in the casing and an end portion of the lid and opposed to the side wall portion, wherein the connecting mechanism is configured such that, during a movement of the lid from the closed position to the open position, a rear end portion of the lid slidingly moves only in a direction from a rear end portion of the main body toward a front end portion thereof or moves in the direction from the rear end portion of the main body toward the front end portion thereof and in an upward direction.