A panel structure includes a large door and a small door. The large door and the small door can be opened and closed. The opening direction of the small door is different from that of the large door. The small door is set in a place which has a plurality of installed openings for installing any type of CD-ROM drive or hard disk. Hence, a user can assemble, disassemble, or replace a CD-ROM drive or a hard disk by simply opening the small door and without opening the large door. Therefore, it is convenient for the user to assemble, disassemble, or replace a CD-ROM drive or a hard disk, insert something into an expanded port, or press any buttons.