There are provided a highly safe epimerase usable in food industry, and a method for producing a ketose. The epimerase is a ketose 3-epimerase obtainable from a microorganism of the genus Arthrobacter, and having the amino acid sequence represented by SEQ ID NO: 1 of the Sequence Listing, and (1) substrate specificity whereby a D- or L-ketose is epimerized at position 3 to produce a corresponding D- or L-ketose, and (2) the highest substrate specificity for D-fructose and D-psicose among D- and L-ketoses. The ketose 3-epimerase is also represented by SEQ ID NO: 3 or SEQ ID NO: 4 of the Sequence Listing, and epimerizes a D- or L-ketose at position 3 to produce a corresponding D- or L-ketose.