Provide is an artificially synthesized antitumor peptide that may suppress proliferation of tumor cells. The peptide provided is a synthetic peptide that contains both (1) an amino acid sequence that forms a signal peptide of a membrane protein, programmed cell death-1 (PD-1), or a modified amino acid sequence thereof in which 1, 2 or 3 amino acid residues deleted from, substituted in or added to the above amino acid sequence; and (2) an amino acid sequence that serves as a cell penetrating peptide (CPP), and wherein the synthetic peptide comprises a total of 100 or fewer amino acid residues.