Provide are a peptide gel with practically sufficient mechanical strength and a self-assembling peptide capable of forming the peptide gel. The self-assembling peptide is formed of the following amino acid sequence: a1b1c1b2a2b3db4a3b5c2b6a4 where: a1 to a4 each represent a basic amino acid residue; b1 to b6 each represent an uncharged polar amino acid residue and/or a hydrophobic amino acid residue, provided that at least five thereof each represent a hydrophobic amino acid residue; c1 and c2 each represent an acidic amino acid residue; and d represents a hydrophobic amino acid residue.