1. A non-transitory, computer-readable recording medium that stores therein a clustering program that causes a computer to execute: setting a plurality of clusters that include a plurality of nodes of a graph, respectively; merging a cluster pair having the maximum bonding strength among all cluster pairs into a merged cluster; calculating the bonding strength between the merged cluster and each of other clusters; and extracting a cluster pair from among clusters after merge when a quantity of the clusters after merge is larger than a given number, wherein the calculating includes calculating the bonding strength f pq between the merged cluster and each of clusters linked thereto based on the following equation: f pq = l ⁡ ( C p , C q ) λ n ⁡ ( C p ) v × n ⁡ ( C q ) v × d ⁡ ( C p ) δ × d ⁡ ( C q ) δ where n(C p ) denotes the number of nodes in a first cluster C p ; n(C q ) denotes the number of nodes in a second cluster C q ; d(C p ) denotes the number of links originating from the first cluster C p ; d(C q ) denotes the number of links originating from the second cluster C q ; l(C p , C q ) denotes the number of links linking together the first and second clusters C p and C q ; and ν, δ, and λ denote adjustment coefficients.