PURPOSE: To perform the maintenance efficiently, by connecting a maintenance device and plural maintenance diagnostic devices through data processing devices for maintenance by a communication line.

CONSTITUTION: When a data processing device 12-1 becomes faulty and a state set command of the device 12-1 is inputted from an input/output part of a maintenance device 21, this command is transmitted to a data processing device 11-1 through a line 4. When receiving this command, the device 11-1 converts this state set command to a state set instruction and transmits this instruction to a maintenance diagnostic device 12-2 through the line 4. The control is transferred to a state read processing in a processing part of the diagnostic device 12-2. Thereafter, state information is transmitted to a data communication center 11 through the line 4. An information processing part edits this information and transmits it from a transmitting part to the maintenance device 21 through the line 4. The operator of the maintenance device 21 watches state information of the faulty device 12-1 for the state set command to determine a state set command to be next supplied.