A method for producing a plant exhibiting an improved biomass level and having a decreased productivity of fats and oils per individual plant comprising steps of: introducing a fusion gene into a plant, wherein the fusion gene codes for a chimeric protein comprising a transcriptional factor comprising any one of the following proteins (a) to (c) and a functional peptide that converts an arbitrary transcriptional factor into a transcriptional repression factor: (a) a protein comprising the amino acid sequence shown in SEQ ID NO: 2; (b) a protein comprising an amino acid sequence that has a deletion, a substitution, an addition, or an insertion of one to twenty amino acids with respect to the amino acid sequence shown in SEQ ID NO: 2 and having activity of accelerating transcription; and (c) a protein encoded by a polynucleotide hybridizing under stringent conditions to a polynucleotide that comprises a nucleotide sequence complementary to the nucleotide sequence shown in SEQ ID NO: 1 and having activity of accelerating transcription: wherein the functional peptide has the amino acid sequence represented by any one of the following formulae to: X1-Leu-Asp-Leu-X2-Leu-X3 wherein X1 denotes 0 to 10 amino acid residues, X2 denotes Asn or Glu, and X3 denotes at least 6 amino acid residues; Y1-Phe-Asp-Leu-Asn-Y2-Y3 wherein Y1 denotes 0 to 10 amino acid residues, Y2 denotes Phe or Ile, and Y3 denotes at least 6 amino acid residues; Z1-Asp-Leu-Z2-Leu-Arg-Leu-Z3 wherein Z1 denotes Leu, Asp-Leu, or Leu-Asp-Leu, Z2 denotes Glu, Gln, or Asp, and Z3 denotes 0 to 10 amino acid residues; Asp-Leu-Z4-Leu-Arg-Leu wherein Z4 denotes Glu, Gln, or Asp; α1-Leu-β1-Leu-γ1-Leu; α1-Leu-β1-Leu-γ2-Leu; α1-Leu-β2-Leu-Arg-Leu; and α2-Leu-β2-Leu-Arg-Leu and wherein in the formulae to, α1 denotes Asp, Asn, Glu, Gln, Thr, or Ser, α2 denotes Asn, Glu, Gln, Thr, or Ser, β1 denotes Asp, Gln, Asn, Arg, Glu, Thr, Ser, or His, β2 denotes Asn, Arg, Thr, Ser, or His, γ1 denotes Arg, Gln, Asn, Thr, Ser, His, Lys, or Asp, and γ2 denotes Gln, Asn, Thr, Ser, His, Lys, or Asp; and selecting a plant having the introduced fusion gene, exhibiting an improved biomass level and having a decreased productivity of fats and oils. introducing a fusion gene into a plant, wherein the fusion gene codes for a chimeric protein comprising a transcriptional factor comprising any one of the following proteins (a) to (c) and a functional peptide that converts an arbitrary transcriptional factor into a transcriptional repression factor: (a) a protein comprising the amino acid sequence shown in SEQ ID NO: 2; (b) a protein comprising an amino acid sequence that has a deletion, a substitution, an addition, or an insertion of one to twenty amino acids with respect to the amino acid sequence shown in SEQ ID NO: 2 and having activity of accelerating transcription; and (c) a protein encoded by a polynucleotide hybridizing under stringent conditions to a polynucleotide that comprises a nucleotide sequence complementary to the nucleotide sequence shown in SEQ ID NO: 1 and having activity of accelerating transcription: (a) a protein comprising the amino acid sequence shown in SEQ ID NO: 2; (b) a protein comprising an amino acid sequence that has a deletion, a substitution, an addition, or an insertion of one to twenty amino acids with respect to the amino acid sequence shown in SEQ ID NO: 2 and having activity of accelerating transcription; and (c) a protein encoded by a polynucleotide hybridizing under stringent conditions to a polynucleotide that comprises a nucleotide sequence complementary to the nucleotide sequence shown in SEQ ID NO: 1 and having activity of accelerating transcription: wherein the functional peptide has the amino acid sequence represented by any one of the following formulae to: X1-Leu-Asp-Leu-X2-Leu-X3 wherein X1 denotes 0 to 10 amino acid residues, X2 denotes Asn or Glu, and X3 denotes at least 6 amino acid residues; Y1-Phe-Asp-Leu-Asn-Y2-Y3 wherein Y1 denotes 0 to 10 amino acid residues, Y2 denotes Phe or Ile, and Y3 denotes at least 6 amino acid residues; Z1-Asp-Leu-Z2-Leu-Arg-Leu-Z3 wherein Z1 denotes Leu, Asp-Leu, or Leu-Asp-Leu, Z2 denotes Glu, Gln, or Asp, and Z3 denotes 0 to 10 amino acid residues; Asp-Leu-Z4-Leu-Arg-Leu wherein Z4 denotes Glu, Gln, or Asp; α1-Leu-β1-Leu-γ1-Leu; α1-Leu-β1-Leu-γ2-Leu; α1-Leu-β2-Leu-Arg-Leu; and α2-Leu-β2-Leu-Arg-Leu and wherein in the formulae to, α1 denotes Asp, Asn, Glu, Gln, Thr, or Ser, α2 denotes Asn, Glu, Gln, Thr, or Ser, β1 denotes Asp, Gln, Asn, Arg, Glu, Thr, Ser, or His, β2 denotes Asn, Arg, Thr, Ser, or His, γ1 denotes Arg, Gln, Asn, Thr, Ser, His, Lys, or Asp, and γ2 denotes Gln, Asn, Thr, Ser, His, Lys, or Asp; and selecting a plant having the introduced fusion gene, exhibiting an improved biomass level and having a decreased productivity of fats and oils.