An assembly structure low in height and suitable for downsizing and for high hermeticity sealing in a micro hard disk drive is provided. Burrings projecting toward a top cover and having an internal thread cut are formed on 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. Burrings are formed extendedly upward on flat portions of recesses at certain corners of the top cover. These 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.