An I/O expansion device for additional inputs and outputs, and an apparatus and a method for applying this device. The device includes a connector (CN1) for the connection with a parallel port, a plurality of 4-bit input/output ports, and an I/O expansion circuit (100) having a 4-bit data bus for data transfer between a plurality of 4-bit input/output ports and the connector. The I/O expansion circuit (100) includes a control input CTRL including a strobe PROG for controlling fetch of the command for selecting the input/output ports and an operation mode, a data input DIN that recieves a command in accordance with the state transition of the strobe PROG and data to be transferred to the input/output port selected by the command and a data output DOUT for giving the state of the selected port through the connector.