Efficiency of communication to a root device is improved. A broadcast hello packet including a hierarchical number is received; the hello packets of one or more devices are collected and an own-device-hierarchical-number is set to a value by adding one to the smallest number of the hierarchical numbers; each device having a hierarchical number smaller than the own-device-hierarchical-number by one is retained as a parent device; each device having the same hierarchical number as the own-device-hierarchical-number is retained as a sibling device; each device having a hierarchical number larger than the own-device-hierarchical-number by one or more is retained as a child device; an indication of the retaining as the parent device, as the sibling device, and as the child device are each transmitted so as to reach the parent device, the sibling device, and the child device; and a hello packet including the own-device hierarchical number is broadcast.