A first information processor transmits a bubble packet to a second communication control unit for leaving transmission record in a first communication control unit by way of the first communication control unit, a second information processor transmits a reply packet to one or more ports including at least the bubble packet transmitting port as the port of the first communication control unit used in transmission of bubble packet, and the first information processor receives the reply packet transmitted from the second information processor by way of the second communication control unit. In this configuration, the invention presents a communication system capable of establishing communication between plural information processors for communicating by way of communication control unit (NAT).