The present invention relates to a novel SGR (STAY-GREEN) gene participating in chlorophyll catabolism during plant senescence, thereby causing leaf yellowing, a method for preparing stay-green transgenic plants maintaining greenness during leaf senescence, which being characterized by mutating the SGR gene, suppressing the expression of the SGR gene, or deactivating the SGR protein which is encoded by the SGR gene, and stay-green mutant plants produced by said method. According to the present invention, leaf greenness can be maintained for a long time by mutating the SGR gene, suppressing the expression of the SGR gene, or inactivating the protein encoded by the SGR gene, thereby preventing leaf yellowing of plants in yellowing plant caused by chlorophyll catabolism during leaf senescence.