This invention is intended to identify a gene that regulates endoreduplication in a plant and to use such gene for breeding aimed at increasing the crop size. This invention provides a gene encoding an Arabidopsis thaliana-derived protein having an amino acid sequence as shown in SEQ ID NO: 2, 4, 6, 8, 10, or 12, which has endoreduplication promoting activity, a transgenic plant into which such gene has been introduced, thereby increasing the nuclear DNA content in the cells of such plant, and a method of using such gene to increase the size of the entire plant or a part thereof.
