PROBLEM TO BE SOLVED: To provide a method for improving fat and oil productivity using a gene having a new function which can increase or decrease substance productivity, especially a fat and oil content of a seed.SOLUTION: The present invention relates to a method where a chimeric protein, in which a transcription factor consisting of a protein which comprises a specific amino acid sequence is fused with a functional peptide which converts any transcription factor into a transcriptional repressor, is expressed in a plant body, and fat and oil productivity per plant body is significantly improved.