To produce a lower alkyl ester of 2-keto-L-gulonic acid (2KGA) useful as a precursor for producing L-ascorbic acid in an energy-saving manner in high yield by crystallizing a monohydrate crystal of the 2KGA into an anhydrous salt crystal and then reacting the resultant anhydrous salt crystal with a lower alcohol.

A monohydrate crystal of 2KGA is crystallized in a water-soluble alcohol solvent to provide an anhydrous salt crystal, which is then reacted with a lower alcohol to produce a lower alkyl ester of the 2KGA with an extremely small amount of the lower alcohol as compared with that of a conventional method. The dehydrating reaction is preferably carried out by charging the 2KGA into a water-soluble alcohol heated at 30-60°C and usually stirring the resultant mixture at the temperature for 30 min to 2 h. In order to increase the yield, the reactional mixture is preferably cooled to ≤10°C, aged for at least 1 h and then subjected to filtration. The above ester can be lactonized to produce a metal L-ascorbate represented by the formula (M is an alkali metal or an alkaline earth metal) in an energy-saving manner.