A PC card adapter and an I/O adapter as expansion devices respectively have discrimination circuits for outputting discrimination signals. A computer body has an expansion connector commonly used for the PC card adapter and the I/O card adapter, and an I/O controller. A selected one of the PC card adapter and the I/O adapter is connected to the expansion connector. The I/O controller has an input/output section commonly used for the PC card adapter and the I/O adapter and corresponding to the expansion connector, and a bus switching circuit for switching bus signal lines for input/output processing of interface signals input/output through the input/output section. With this arrangement, in a computer system using a plurality of types of expansion devices such as a PC card slot, a printer port, and a serial communication port, the number of input/output pins and the number of connectors of each controller for interfacing between a corresponding expansion device and the computer body can be decreased, thereby realizing reductions in the size and cost of the system.