1. A non-transitory computer-readable recording medium storing therein an information searching program of a computer that retrieves a sub graph matching an inquiry graph from a graph to be searched including nodes and a link interlinking the nodes, the information searching program causing the computer to execute: extracting, from among clusters of nodes in the graph to be searched, a plurality of cluster pairs, each of the cluster pairs consisting of a first cluster and a second cluster including a node linked by a link to a node in the first cluster, wherein the clusters include nodes that differ; calculating a bonding strength for each of the cluster pairs extracted at the extracting; determining, among the cluster pairs and based on the bonding strength of each of the cluster pairs, a cluster pair to be merged; merging the cluster pair determined at the determining; narrowing the merged clusters, based on a condition of the inquiry graph; searching merged clusters narrowed at the narrowing, for a sub graph matching the inquiry graph; and outputting a search result of the searching, the calculating includes calculating the bonding strength f pq based on an equation of: 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 the first cluster C p ; n(C q ) denotes the number of nodes in the 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.