According to one embodiment, a wireless mesh network includes a plurality of nodes of which one is a root node and two nodes hierarchically adjacent have a parent-child relationship. In a node in the wireless mesh network, an authentication processing unit executes an authentication for network access with the root node when the node joins in the wireless mesh network. A root key setting unit generates a root key as a common key between the root node and the node. A parent-child key setting unit generates a parent-child key as a common key between a parent node and the node, encrypts the parent-child key using the root key, and sends the parent-child key encrypted to the root node. A group key acquisition unit receives a group key encrypted using the parent-child key from the parent node, and decrypts the group key using the parent-child key.