In an information processing system having a portable terminal unit, such as a hand-held computer, driven by a rechargeable battery and a data communication adaptor for enabling communication between the portable terminal unit and a host computer of the system, the terminal unit and the data communication adaptor are designed to minimize the size thereof in the following five ways. First, a memory card holding mechanism for connecting the memory card of the portable terminal unit is provided between a swinging back lid and a recess of the back casing. Second, a normal interface and a high-speed interface are both provided in the portable terminal unit and the data communication adaptor respectively. Third, an auxiliary connector for an optional device to extend a function of the portable terminal unit is provided in the opening of the casing of the portable terminal unit movable in three dimensions mounted on a printed circuit board. Fourth, a moisture resistant seal made of rubber or resin is molded integrally onto one of the joining edges of an upper casing and a lower casing. Fifth, an electric power source portion of the data communication adaptor is used not only for charging the battery of the portable terminal unit but also supplying a power adequate for enabling a data transmission operation thereof.