Estimating that sd1 gene is a C20 oxidase gene, the rice counterpart of Arabidopsis C20 oxidase gene is isolated and identified. As a result, it is found out that rice sd1 gene is a gene encoded from a novel C20 oxidase gene. As the results of the subsequent studies, it is clarified that mutation in this gene induces plant semidwarfing. It is highly expected that use of plan sd1 gene would enable an increase in the yield of ornamental plants, addition of aesthetic value via dwarfing, an increase in the yield by marker selection and efficient breeding of dwarfed plants.