A peptide having a sequence which satisfies all of the following requirements (I) to (V) and has an activity of stimulating an immune cell: (I) the number of constituent amino acid residues is 12 to 36; (II) the sequence is amphipathic; (III) the charge of the whole molecule is +2 or more; (IV) when constituent amino acid residues are arranged so as to form an a-helical structure, a side chain of any aromatic amino acid residue is not located between side chains of at least two positively charged amino acid residues on a side where hydrophilic amino acid residues are located; and (V) the sequence contains an amino acid residue which serves as a cleavage point for a mitochondrial processing enzyme.