A main case body of a musical instrument, which is capable of securing a degree of freedom of design. A case structure includes a lower front case, a lower rear case, and an upper case. The lower front case has a keyboard unit mounted therein. A bottom part extends in a front-rear direction of the lower front case. A key supporting part is formed such that it rises from the rear of the bottom part of the lower front case and extends in the key arrangement direction. A stopper body for stopping a free end of each key is formed such that it rises from the front of the bottom part of the lower front case and extends in the key arrangement direction. A battery case is formed by six battery terminal-associated parts arranged side by side for batteries between the key support part and the stopper body.