PURPOSE: To prevent the omission of a printing line or the double printing of the same line by backing up and storing the number of busy line buffer of a printer on the side of an ECR main body, and setting the retransmission location of printing data according to the number of busy line buffer memory just before a power failure at the time of recovering the power failure.

CONSTITUTION: The electronic cache register(ECR) main body 1 confirms the number of buffer memory during the time when a printer 2 is busy, by periodically performing the transmission of a status request command and the reception of status data to be returned in response to this. When a power failure is generated, the number of line buffer memory of the printer 2 which was busy just before the power failure is held as status data in the status memory 14 of the ECR main body 1. When the power failure is recovered, the ECR main body 1 updates the reading location of a line buffer memory 11 according to the status data read from a status memory 14 so as to start a retransmission from a printing unterminated line. Thus, the double printing of the same line is prevented.