Radio devices (31 to 43) receive a prescribed number of Hello packets from each of n radio devices existing within one hop from each radio device, detects six pairs of maximum and minimum values MAX[1] and MIN[1] to MAX[6] and MIN[6] based on the plurality of reception signal strengths of the plurality of received Hello packets, detects two adjacent maximum values having the largest difference among six pairs of maximum and minimum values D_MAX[1] and D_MIN[1] to D_MAX[6] and D_MIN[6] when the six maximum values MAX[1] to MAX[6] are in descending order, and determines the average of two minimum values corresponding to the detected two maximum values as a threshold. The radio devices (31 to 43) each produce a routing table including routes having reception signal strengths equal to or more than the threshold.