A mutated tryptophan oxidase suitable for practical implementation is described herein. Specifically, a mutated tryptophan oxidase wherein at least one amino acid residue of a wild-type tryptophan oxidase is mutated and, as a result, has higher tryptophan oxidase activity and/or stability as compared to the wild-type tryptophan oxidase. The mutated tryptophan oxidase can be derived from a wild-type tryptophan oxidase having at least one of Motifs (2), (3), (5), (7), (9), (11), (13), and (14), and at least one amino acid residue in any of these motifs can have mutation. The mutated tryptophan oxidase also can have a mutation of one or more amino acid residues in an amino acid sequence represented by SEQ ID NO: 2 and a sequence homologous thereto.