The antitumor peptide provided by the present invention is a synthetic peptide including both of:(1) an amino acid sequence composing the second transmembrane region from the N-terminal of a membrane protein sphingosine 1-phosphate receptor 1 (S1PR1), or a modified amino acid sequence formed by deletion substitution or addition of 1, 2 or 3 amino acid residues in the amino acid sequence； and(2) an amino acid sequence functioning as a cell-penetrating peptide (CPP)； whereinthe total number of amino acid residues is 100 or less.
