A gene causative of a phenotype is easily specified and thus gene functions are comprehensively analyzed. A method for analyzing a gene function includes (a) infecting a plant group with a full-length cDNA library containing an expression regulatory sequence to introduce the cDNA into the plants; (b) selecting a plant complying with desired selection requirements from the plants into which the cDNA has been introduced; (c) isolating the cDNA from the selected plant; and (d) re-introducing the thus isolated cDNA into a plant of the same species as the plant into which the cDNA has been introduced, to reconfirm a phenotype according to the selection requirements.