In a computer system, when an expansion unit is connected to a computer main body (1) by an interface card (3), power supply control and I/O port selection are performed at the start of the computer system. In the power supply control, when an expansion power supply of the expansion unit is turned on, a first voltage is supplied to a main power supply (20) of the computer main body (1). When the main power supply (20) is turned on, main voltages are supplied to computer main body elements from the main power supply (20) in response to the main voltage, and a second voltage is supplied to the expansion power supply through the interface (3). In response to the second voltage, expansion voltages is supplied to expansion unit elements, and an interface voltage is supplied to the interface (3). After the power supply control is completed, the I/O port selection is performed. The port addresses of the I/O ports in the computer main body (1) are compared with the port addresses of the I/O ports in the expansion unit. If both port addresses coincide with each other, the I/O ports in the computer main body (1) are disabled, and the I/O ports in the expansion unit are enabled.