To obtain a (1-10C)alkyl 2-keto-L-gulonate as an intermediate for synthesizing L-ascorbic acid (vitamin C) in a sufficiently low residual water content rate and a short esterification reaction residence time.

In the method for producing the (1-10C)alkyl 2-keto-L-gulonate by esterifying 2-keto-L-gulonic acid or diacetone-2-keto-L-gulonic acid with a 1-10C alcohol in the presence of an acidic catalyst, esterification is carried out in the hot surface of a liquid film while removing water at the same time. Solid is not formed in a reactor though keto-L-gulonic acid is difficultly soluble in an lower alcohol such as methanol, ethanol, propanol, butanol or pentanol and a produced ester is present in a state not containing a solvent after removing water.