PURPOSE: To prevent a manufacturing cost from increasing and an operating speed from decreasing by preventing coming noise to be superposed on a printing data input signal from invading into a printer and external leakage of noise to be superposed on response data output signal in the printer.

CONSTITUTION: A first electrooptical converter 130a of a printer converts a printing data input signal 101 into an optical signal, a light transmission medium 160 transmits the optical signal output to a first optoelectric converter 130b. The converter 103b converts the output transmitted from the medium 160 into an electric signal, and inputs it to a printing circuit 900. On the other hand, a second electrooptical converter 230a converts response data of the circuit 900 into an optical signal, and a light transmission medium 260 transmits the output to a second optoelectric converter 230b. The converter 230b the output transmitted from the medium 260 into an electric signal, and generates a response data output signal 202. Accordingly, special means for preventing invasion of coming noise and leakage of internal noise is not required to avoid an increase in a manufacturing cost and a decrease in an operating speed.