Abstract: Provided are a synthetic peptide having a multinucleation-inducing activity that can cause multinucleation in a target cell, a multinucleation-inducing agent containing this peptide, and a multinucleation induction method that uses this peptide, The synthetic peptide provided by the present invention is an artificially synthesized peptide that contains (A) a membrane-permeable peptide sequence and (B) a multinucleation-inducing peptide sequence, and that can cause multinucleation in at least one type of eukaryotic cell of human origin Or non-human mammal origin. The multinucleation-inducing peptide sequence is constituted of an amino acid sequence given by SEQ ID NO: 1 or SEQ ID NO: 2 or a modified amino acid sequence therefrom.