1. A node apparatus in a network including a plurality of network apparatuses, the node apparatus comprising: a cluster information memory configured to store apparatus identification information of node apparatus included in a first cluster, which is a group of node apparatuses route information of which is stored by the node apparatus, among the plurality of node apparatuses in the network; a reception unit configured to receive, from adjacent node apparatus, a hello packet used to notify route information about a route for conveying information in the network, and apparatus identification information of node apparatus included in a second cluster, which is a group of node apparatuses route information of which is stored by the node apparatus; a cluster node change verification unit configured to verify that the number of node apparatuses included in the first cluster is equal to or lower than a specified ratio to a maximum number of cluster nodes defined with a specified integer value, and that the number does not change for a specified amount of time; a merge cluster node number determination unit configured to obtain the number of node apparatuses included in the first cluster based on the apparatus identification information of the node apparatuses, which is stored in the cluster information memory, upon receipt of the hello packet from one of adjacent node apparatuses included in the second cluster different from the first cluster, and to determine whether a sum of the number of node apparatuses included in the first cluster and the number of node apparatuses included in the second cluster is equal to or smaller than the maximum number of cluster nodes; a cluster merge processing unit configured to update the apparatus identification information stored in the cluster information memory by adding the apparatus identification information of the node apparatuses included in the second cluster to the apparatus identification information of the node apparatuses included in the first cluster based on a result of the determination of the merge cluster node number determination unit, and to merge the second cluster with the first cluster; a hello packet generation unit configured to generate a different hello packet by referencing the updated apparatus identification information of the node apparatuses, which is stored in the cluster information memory; and a transmission unit configured to transmit the different hello packet.