Abstract: It has been found that introducing into a Poaceae plant an Hd3a gene, which is a flower-bud-formation inducing gene, positioned downstream of a promoter whose expression is induced by a plant activator treatment makes it possible to control the flowering time of the Poaceae plant in accordance with a plant activator treatment timing. It has been found that further introducing a Ghd7 gene, which functions to suppress flower bud formation, into the plant makes it possible to suppress the expression of an endogenous Hd3a gene and increase the efficiency of controlling the flowering time.