To provide a new fructosylamine oxidase reactive with fructosylvaline, and to provide a method for producing the oxidase and a method for assaying the oxidase.

The fructosylamine oxidase is produced by culturing a microorganism belonging to Arthrobacter sp., or a transformant obtained by transforming a microorganism by a recombinant vector obtained by incorporating a DNA fragment containing a gene encoding the fructosylamine oxidase. The produced fructosylamine oxidase is specific to the fructosylvaline, and the kit sensor using the oxidase is useful for measuring glycosylated hemoglobin.