A mutant glycine oxidase is obtained by substituting at least one wild-type amino acid sequence derived from thermophilic bacteria belonging to the family Bacillus with another amino acid, and has the following enzyme properties. Molecular weight: 40,000±2,000 daltons by SDS-PAGE. Optimum temperature: 45° C. under the condition of pH 8.5 in presence of pyrophosphate. Optimum pH: pH 8.0 under the condition of 37° C. in presence of pyrophosphate. Thermal stability: Stable up to 70° C. under the condition of pH 8.5 while retaining for 1 hour in presence of pyrophosphate. pH Stability: Stable in the range of pH 5.5 to 10.0 under the condition of 4° C. while retaining for 24 hours in presence of pyrophosphate. Specific activity: 1.2 units/mg or more. Kinetic constant Km: 0.2 mM or less.