This invention provides an antitumor peptide and an antitumor composition comprising the peptide for suppressing proliferation of at least one species of tumor cells. The antitumor composition provided by this invention comprises an antitumor peptide capable of inducing formation of multipolar spindles in at least one species of tumor cells, and at least one species of pharmaceutically acceptable carrier, wherein the antitumor peptide is a synthetic peptide having an amino acid sequence selected from SEQ ID NOs: 1 to 22 or an amino acid sequence formed by substituting, deleting and/or adding one, two or three amino acid residues in/from/to the selected amino acid sequence.
