A musical instrument body is provided with an array of keys at a front portion of the musical instrument body, and side plates at both side portions of the musical instrument body. A keyboard lid is mounted in the musical instrument body slidably between a closed position for covering the array of the keys and an open position for exposing the array of the keys. In case that the keyboard lid is shifted to the open position from the closed position, a front lid section moves rearward along a first guide recess together with a rear lid section, and when the rear lid section reaches a rearmost position, a rear lid supporting part is positioned in a second guide recess, while a front lid supporting part moves from the first guide recess to a third guide recess at a branch point according to release of connection between the front lid section and the rear lid section, and subsequently the front lid section is guided along the third guide portion.