There are demands for an assembly structure which is low in height and is suitable for downsizing and for a sealing structure of high hermeticity in a micro hard disk drive. In one embodiment, a burring projecting toward a top cover and having an internal thread cut on its inner wall is formed at each of regions (flat portions in steps) of a base plate where the top cover is screw-fastened to the base plate, the base plate accommodating a functional unit such as a disk and a head. A burring is formed extendedly upward on each of flat portions of recesses qt three corners formed by pressing on the top cover. The burrings have a diameter larger than that of the burrings of the base plate, the burrings of the base plate are fitted into the burrings to be screw-fastened with top cover fastening screws. After attaching the top cover to the base plate, a covering seal is applied to cover the respective top faces of the top cover, base plate, and the top cover screws.