A first information processor transmits a bubble packet to a port assigned after assignment of specified port from reference port in a second communication control unit, a server detects the position of the bubble packet transmitting port in a first communication control unit used in transmission of this bubble packet, and a second information processor transmits a reply packet to the detected bubble port transmitting port. In this configuration, the invention presents a communication system capable of establishing more securely communication between plural information processors for communicating by way of communication control unit (NAT). In this configuration, the invention presents a communication system capable of establishing more securely communication between plural information processors for communicating by way of communication control unit (NAT).