PURPOSE: To obtain the titled substance useful as a precursor for ascorbic acid economically in high yield, by oxidizing diacetone-L-sorbose with an oxygen- containing gas in the presence of a catalyst prepared by adding Pb, Bi, etc. to Pd and/or Pt.

CONSTITUTION: In preparing the titled substance by oxidizing diacetone-L-sorbose, a catalyst obtained by adding Pb and/or Bi to Pd and/or Pt is used, to oxidize it with an oxygen-containing gas (e.g., oxygen, air). The catalyst is usually supported by active carbon, alumina, etc. for use, the supported amount of Pd and Pt is 0.5W5wt% in the state of metal or oxide, and that of Pb and Bi is 1W5wt% in the state of metal, hydroxide, oxide, etc. In the example of preparing the catalyst, active carbon powder is soaked in an aqueous solution of chloroplatinic acid and lead nitrate, and reduced with an aqueous solution of formalin.