A relay device 40, when it relays data blocks transmitted in succession from a transmitter device 20 to a receiver device 10, is caused to identify a state code showing a state of a communication resource used when the data blocks are transferred. Relay device 40 divides the identified state code into portions of a predetermined data size and writes the divided portions into a predetermined area of the data blocks. Receiver device 10 transmits to transmitter device 20 an acknowledgement containing the same data as is written in a predetermined area of the received data block. Transmitter device 20, upon receipt of the acknowledgement, reads the state code that is divided and written in the acknowledgement and increases or reduces the number of data blocks on the basis of the state code.