A novel fructosyl amino acid oxidase derived from genus Fusarium or Gibberella, a process for producing the enzyme, an assay of an amadori compound using the enzyme, a reagent or a kit containing the enzyme, a process for producing fructosyl lysine and/or fructosyl N.sup..alpha. -Z-lysine as a substrate of a fructosyl amino acid oxidase, which is useful for screening and/or culturing a microorganism capable of producing the enzyme, and a medium containing fructosyl lysine and/or fructosyl N.sup..alpha. -Z-lysine for screening and/or culturing a microorganism capable of producing a fructosyl amino acid oxidase is provided.