A music stand device can be securely mounted in a keyboard instrument or the like. A music score plate holds a musical score. A mounting member is mounted on a roof plate. A fixing member is fixed to the musical score plate and has one end thereof rotatably supported by one end of the mounting member. A supporting member has one end thereof rotatably supported by the other end of the fixing member and the other end thereof engaged by a part of the mounting member to thereby maintain the musical score plate in the standing position. An engaging bolt is provided in the roof plate and located in opposed relation to the mounting member. The mounting member is slid to engage an engaging hole and the engaging bolt with each other, with the mounting member fastened to the roof plate by a fastening tool from above through fastening holes.