The present inventors presumed that the sd1 gene was the C20 oxidase gene, and isolated and identified the rice counterpart of Arbidopsis C20 oxidase gene. As a result, it was revealed that the rice sd1 gene encodes a novel C20 oxidase. Further studies showed that mutation in this gene led to plant semidwarfing. Utilization of the plant sd1 gene is expected to increase the yield of plants, particularly useful agricultural crops and ornamental plants, to add aesthetic value to ornamental plants through dwarfing, and also to increase the yield and efficient breeding of dwarfed plants by marker selection.