A method for increasing seed protein content in a plant by causing expression of a chimeric protein obtained by fusing a transcription factor consisting of any one of the following proteins (a) to (c) and a functional peptide capable of converting the transcription factor of any one of the following proteins (a) to (c) into a transcriptional repressor in a plant: (a) a protein comprising the amino acid sequence of SEQ ID NO: 4; (b) a protein having transactivation activity and comprising an amino acid sequence that has a deletion, a substitution, an addition, or an insertion of 1 to 10 amino acids with respect to the amino acid sequence of SEQ ID NO: 4; and (c) a protein having transactivation activity encoded by a polynucleotide that hybridizes under stringent conditions to a polynucleotide consisting of a nucleotide sequence complementary to the nucleotide sequence of SEQ ID NO: 3; wherein the functional peptide has an amino acid sequence represented by any one of the following formulae to: X1-Leu-Asp-Leu-X2-Leu-X3 (where X1 denotes a set of 0 to 10 amino acid residues, X2 denotes Asn or Glu, and X3 denotes a set of at least 6 amino acid residues); Y1-Phe-Asp-Leu-Asn-Y2-Y3 (where Y1 denotes a set of 0 to 10 amino acid residues, Y2 denotes Phe or Ile, and Y3 denotes a set of at least 6 amino acid residues); Z1-Asp-Leu-Z2-Leu-Arg-Leu-Z3 (where Z1 denotes Leu, Asp-Leu, or Leu-Asp-Leu, Z2 denotes Glu, Gln, or Asp, and Z3 denotes a set of 0 to 10 amino acid residues); Asp-Leu-Z4-Leu-Arg-Leu (where 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-β1-Leu-Arg-Leu (where 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 in formulae to). (a) a protein comprising the amino acid sequence of SEQ ID NO: 4; (b) a protein having transactivation activity and comprising an amino acid sequence that has a deletion, a substitution, an addition, or an insertion of 1 to 10 amino acids with respect to the amino acid sequence of SEQ ID NO: 4; and (c) a protein having transactivation activity encoded by a polynucleotide that hybridizes under stringent conditions to a polynucleotide consisting of a nucleotide sequence complementary to the nucleotide sequence of SEQ ID NO: 3; wherein the functional peptide has an amino acid sequence represented by any one of the following formulae to: X1-Leu-Asp-Leu-X2-Leu-X3 (where X1 denotes a set of 0 to 10 amino acid residues, X2 denotes Asn or Glu, and X3 denotes a set of at least 6 amino acid residues); Y1-Phe-Asp-Leu-Asn-Y2-Y3 (where Y1 denotes a set of 0 to 10 amino acid residues, Y2 denotes Phe or Ile, and Y3 denotes a set of at least 6 amino acid residues); Z1-Asp-Leu-Z2-Leu-Arg-Leu-Z3 (where Z1 denotes Leu, Asp-Leu, or Leu-Asp-Leu, Z2 denotes Glu, Gln, or Asp, and Z3 denotes a set of 0 to 10 amino acid residues); Asp-Leu-Z4-Leu-Arg-Leu (where 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-β1-Leu-Arg-Leu (where 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 in formulae to). X1-Leu-Asp-Leu-X2-Leu-X3 (where X1 denotes a set of 0 to 10 amino acid residues, X2 denotes Asn or Glu, and X3 denotes a set of at least 6 amino acid residues); Y1-Phe-Asp-Leu-Asn-Y2-Y3 (where Y1 denotes a set of 0 to 10 amino acid residues, Y2 denotes Phe or Ile, and Y3 denotes a set of at least 6 amino acid residues); Z1-Asp-Leu-Z2-Leu-Arg-Leu-Z3 (where Z1 denotes Leu, Asp-Leu, or Leu-Asp-Leu, Z2 denotes Glu, Gln, or Asp, and Z3 denotes a set of 0 to 10 amino acid residues); Asp-Leu-Z4-Leu-Arg-Leu (where 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-β1-Leu-Arg-Leu