This invention provides a novel Fas antigen derivative which comprises at least a part or entire portion of Fas antigen extracellular region polypeptide in which at least one amino acid residue is deleted from a group of amino acid residues starting from the N-terminal amino acid residue of the Fas antigen polypeptide to a cysteine residue most close to the N-terminal side (excluding said cysteine residue), as well as a DNA fragment which encodes said Fas antigen derivative, a recombinant DNA molecule which contains said DNA sequence, a transformant in which said recombinant DNA molecule is introduced, a method for the production of said Fas antigen derivative, a medicament which contains said novel Fas antigen derivative as the active ingredient and a method for the improvement of activities and functions of Fas antigen and the like.