A wireless mesh network includes a plurality of nodes to which a device key is assigned. The device key belongs to one of a plurality of groups. In a root node, a correspondence relationship between the nodes and the device key thereof, and a correspondence relationship between past join nodes and a device key thereof, are stored. When a new node in the wireless mesh network is detected as a past join node, the device key assigned to the past join node is assigned to the new node again. When the new node is not the past join node, a new device key is assigned to the new node. A cipher text is generated by encrypting a message using device keys assigned to the nodes and the new node. If the number of groups to which the device keys belong is fewer, a size of the cipher text is smaller.