The invention provides streptolysin O containing a polypeptide comprising an amino acid sequence having deletion of a polypeptide extending from a lysine residue at position 2 of SEQ ID NO: 1 to any of an alanine residue at position 31, a glutamate residue at position 32, a serine residue at position 33, and an asparagine residue at position 34, and also having deletion of any of an isoleucine residue at position 465 to an alanine residue at position 472 of SEQ ID NO: 1 and all of the subsequent amino acid residues, as well as variants thereof and related DNA, vectors, transformants, and methods of use thereof.