1. A 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; searching merged clusters formed at the merging, for a sub graph matching the inquiry graph; and outputting a search result of the searching.