The invention provides a data transfer apparatus which makes it possible to perform communication between an apparatus connected to a serial communication system via a communication line and a terminal device provided with an infrared communication unit. A data transfer device is set such that an upstream data transmitter and a downstream data transmitter are connected to each other via the data transfer device. In this state, in response to a command input via a keyboard or a mouse, data to be printed is transmitted to a printing apparatus via the data transfer apparatus. The data transfer apparatus receives at its light receiving device an infrared signal from a personal computer. An infrared communication controller derives the content of data. A controller checks whether the printing apparatus is a busy state or not. If the printing apparatus is available, then the controller switches the state of the data transfer device into a state in which the downstream data transmitter and the coder/decoder are connected to each other. After that, data transfer apparatus receives the data to be printed from the personal computer via an infrared ray, and the received data is transferred to the printing apparatus via the light receiving device, an infrared communication controller, a format converter, a coder/decoder, the data transfer device, the downstream data transmitter, and the a pair of signal lines of a communication line. According to the received data, the printing apparatus performs a printing operation.