Abstract: In a simultaneous communication as in a POS system in which the same data including a multiplicity of data blocks is transmitted from a controller to a multiplicity of terminals all at once, the controller transmits all of the data blocks to the terminals without receiving any acknowledgement from each of the terminals in the course of data transmission. Each time the data block is received, the terminals hold information concerning the acknowledgement of the data block. After the termination of transmission of the whole of the data, the controller receives the acknowledgement information from each terminal into a management table. Based on the contents of the mangement table, the controller retransmits the data to a terminal(s) indicative of a negative acknowledgement in a similar manner to the above-mentioned data transmission or while taking acknowledgement for each data block.