A protein comprising an amino acid sequence having at least one mutation selected from a Gly-4 to Ala mutation, a Glu-6 to His mutation, a Ser-14 to Thr mutation, an Ala-37 to Thr or Arg mutation, a Pro-50 to Gln mutation, a Glu-67 to Gly mutation, an Asp-80 to Tyr mutation, a Val-93 to Met mutation, an Arg-156 to Pro mutation, a Leu-164 to Met mutation, an Asn-202 to Asp mutation, a Thr-235 to Ala mutation, an Asn-348 to Tyr mutation, a Gly-362 to Arg mutation and a Val-473 to Ala mutation in the amino acid sequence depicted in SEQ II NO:4. (2) A thermostable protein which comprises an amino acid sequence derived from the amino acid sequence having at least one variation described in (1) and having 1,5-anhydroglucitol dehydrogenase activity. These proteins act specifically on 1,5-anhydroglucitol (1,5-AG), have thermal stability and exhibit excellent storage stability.