The present invention provides a gateway unit which comprises a detecting unit for detecting the congestion from an RM cell from the ATM cell, for discriminating a TCP data gram (TCPDG) from a packet received during the congestion and for acquiring a sequence number of the TCPDG, a sending unit for sending an ACK having a window-controlled value to a transmitting terminal so as to limit a data transmission, and a discarding unit for discarding the packet from the transmitting terminal during the congestion after the confirmation of packet. During the congestion in the ATM network, a pseudo acknowledgment is sent from the gateway unit so as to indicate that the receiving terminal cannot receive data. This restrains the transmission of TCPDG from the transmitting terminal.