PURPOSE: To print a printing data after an electric conduction by an on-line system, by saving the printing data which has been sent to a printing device at the time of a service interruption.

CONSTITUTION: When a commercial power source is on, an electric conduction detector 7 detects a fact that a power source line 6 is in an electric conduction state, and informs it to an on-line device 1. Also, when the commercial power source has become off due to a service interruption, etc., a typewriter (TR) 2 stops its operation, but a power source is supplied from a no-break power unit 3 and the on-line device 1 maintains its operation. In this case, the electric conduction detector 7 detects a fact that the power source line 6 has become a service interruption, and informs it to the on-line device 1. In this way, when the TR 2 is conducted electrically in a step S 102, the step proceeds to S 103, and the TR 2 executes printing through a data line 4. When it is decided to be in the course of a service interruption in the step S 102, a data to be printed is accumulated in a saved-area, in a step S 104, the processing is ended in a step S 105, and after the electric conduction, printing is executed.