Provided are a communication system and the like capable of making a stable relay route even when a propagation channel is fluctuated by the fluctuation of RSSI. For making a tree-type communication route whose root is a core node, each slave node, after the reception of the latest reset routing packet from a node, judges if the transmission source node should be the parent of the tree-type structure based on the sequential average of the reception power of n routing packets received from the same transmission source node. As the reception power is log-normally distributed in general, an asymptotically stable relay route can be obtained, especially by using the sequential average.