PURPOSE: To improve the reliability of a data communication by recognizing an alarm state by a slave alarm recognizing part, informing it to a host device.

CONSTITUTION: A slave alarm recognizing part 12 recognizes an alarm state of a printer side, for instance, paper end, thermal alarm, printer local, power source disconnection, etc., and informs them to a host device 2 by a control code determined in advance. Also, a host communication alarm recognizing part 13 monitors data received from the hose device 2, and prevents the generation of a data omission. In this state, the alarm contents recognized by the host communication alarm recognizing part 13 are displayed on a display LED of an alarm information display part 14. In such a way, in the host device 2, an alarm state of the printer can be known, and an error of a data communication can be recognized.