Abstract: An object is to provide a peptide gelling agent which gels under physiological conditions and which has a relatively short chain length, and a sustained-release gel based on it. Provided is a hydrogelling self-assembling peptide comprising one or two core peptides consisting of an amino acid sequence represented by the following formula: Xaa-Yaa-Zaa-Yaa-Xaa-Yaa-Zaa-Yaa-Xaa, wherein Xaa is independently Ile or Met, Yaa is independently Asp, Glu, Lys, or Arg, and Zaa is independently Ala or Gly, and wherein the full length of the amino acid sequence constituting said self-assembling peptide is 25 amino acids or less.