When a proxy device is used between a transmission terminal and a reception terminal, receives final data from the transmission terminal, and fails immediately thereafter, a situation is avoided where transmission is completed in the transmission terminal, but reception is not completed in the reception terminal. There is provided a means for not returning ACK when a data packet is received in a state in which there is no data which waits for ACK and transmission of which is in progress, and there is no data which waits for arrangement and reception of which is in progress in the proxy device, for returning ACK for data received one step before when a data packet is received in a state in which there is data which waits for ACK and transmission of which is in progress, or there is data which waits for arrangement and reception of which is in progress.