PURPOSE: To make it possible to input information from a power system until input information of master and slave stations matches each other when input information of the master and slave stations out of parent stations is different each other.

CONSTITUTION: When a condition is changed in a power system 10, information of the condition change is transmitted from a child station 12 to parent stations 20 and 22 through a HDLC line 14. In this case, the parent station 20 sends a request for telling receiving information to the parent station 22 every input information, and the parent station 22 sends receiving information to the parent station 20. When receiving information of each parent station is different, the parent station 20 transmits information for resending information of the condition change to the child station 12 regarded as a receiving error, and information of the same condition change as information previously transmitted from the child station 12 is again transmitted to each parent station 20 and 22. This processing is repeated until receiving information of each parent station 20 and 22 matches each other.