A peptide dimer comprising a peptide consisting of the amino acid sequence of SEQ ID NO: 1 and a peptide consisting of any one of the amino acid sequences of SEQ ID NOS: 2 to 4, wherein the peptide consisting of the amino acid sequence of SEQ ID NO: 1 and the peptide consisting of any one of the amino acid sequences of SEQ ID NOS: 2 to 4 are bound through a disulfide bond between a cysteine residue of each peptide is used as an active ingredient of an agent for promoting proliferation of one or more bacterium selected from the group consisting of Bifidobacterium infantis and Bifidobacterium breve.