Abstract: An object of the present invention is to provide a peptide with superior immuno-enhancing activities, which comprises particular amino acid sequence, and an immuno-enhancing agent comprising such a peptide. The present invention relates to the immuno-enhancing agent comprising a peptide consisting of the following amino acid sequence (I): Q1-SerP-X-SerP-Q2 (I); wherein, SerP represents the phosphoserine residue, X represents one to three of any amino acid residues, and either of Q1 or Q2 dose not exist or neither of them exists, or Q1 and Q2 are independently at least one of any amino acid residue.