The present invention aims to provide a medical material which is safe for the live body, has high biocompatibility and is useful for promotion of cell differentiation. The present invention produces a medical material for promoting cell differentiation, which contains polyamino acid as a main component, wherein the polyamino acid contains at least one kind of amino acid residue selected from the group consisting of an alanine residue, a valine residue, a leucine residue, an isoleucine residue, a phenylalanine residue, a glycine residue, a glutamine residue, an aspartic acid residue optionally containing a protecting group in the side chain, a tyrosine residue optionally containing a protecting group in the side chain, a tryptophan residue optionally containing a protecting group in the side chain, a lysine residue optionally containing a protecting group in the side chain, and a glutamic acid residue optionally containing a protecting group in the side chain.