In a computer system constituted by a portable computer and a deskstation used to expand the function of the portable computer, the portable computer has a communication connector used for communication with the deskstation, in addition to a connector for connecting a system bus in the portable computer to the deskstation. Similarly, the deskstation also has a communication connector. When the portable computer is set at a predetermined position of the deskstation, these connectors are connected to each other before the system bus is connected to an expansion device of the deskstation. By using the communication connectors to perform communication between the portable computer and the deskstation, disconnection between the system bus and an expansion connector or forced suspend processing of the portable computer is executed before the portable computer in a power ON state is mounted in the deskstation. In addition, a lock mechanism is provided to lock the portable computer with the deskstation. The lock mechanism is operated/released by registering/deleting a password. Even when a power supply unit having a plurality of power supply outlets is externally provided to the deskstation, processing for sequentially enabling the power supply outlets is executed. Temperature control in the portable computer is also executed.