A first information-processing device at a communication source, is equipped with span of packet life setting part and bubble packet transmitter. The span of packet life setting part sets a span of packet life in a range where a bubble packet transmitted for leaving a transmission history in a NAT at the communication source so that a packet transmitted from the information-processing device at the communication destination, does not reach the information-processing device at the communication destination. Meanwhile, bubble packet transmitter transmits a bubble packet having the span of packet life set by span of packet life setting part through the NAT at the communication source. With such a configuration, the present invention provides an information-processing device that appropriately establishes communications through a NAT with high-level security.