The present invention provides a method for searching for or identifying a useful gene logically, systematically, and efficiently in an extremely short time without largely relying on searcher's knowledge, experience, or the like and even without sequentially conducting gene disruption experiments as in conventional techniques of searching for a useful gene. The present invention also provides an apparatus for the method. Virtual gene clusters each comprising two or more genes are individually scored by summing the respective pieces of differential expression information (obtained using, for example, microarrays) of genomic genes on a per-cluster basis. On the basis of the obtained scores of the virtual gene clusters, a gene cluster containing a useful gene and the useful gene contained in the cluster are searched for.