To provide a new protein (including L-amino acid oxidase of mammal) having L-amino acid oxidase activity by isolating/identifying the L-amino acid oxidase of mammal and to provide an antimicrobial/sterilizing composition utilizing antimicrobial action or sterilization of hydrogen peroxide produced by the protein.

The antimicrobial/sterilizing composition comprises a protein composed of a specific amino acid sequence derived from a mouse or a protein composed of an amino acid sequence in which one or a plurality of amino acids are deleted, substituted or added in the amino acid sequence and having L-amino acid oxidase activity as a hydrogen peroxide-feeding component.