PURPOSE: To industrially advantageously obtain the subject compound used as a raw material for producing vanillin or as an acid catalyst by reacting an enzyme derived from a microorganism having ability to convert glycolic acid into glyoxylic acid or the microorganism with a substrate and then collecting the product.

CONSTITUTION: An enzyme (e.g. glycerol oxidase) derived from a microorganism [e.g. Aspergillus japonicus KY-45 (ATCC-No. 1042)] having an ability to convert from glycolic acid into glyoxylic acid or a microorganism producing this enzyme is added to a buffer solution containing 2.5M sodium glycolate and the enzyme or the microorganism is reacted with the substrate at 25°C for 3-6hr and glycolic acid which is the substrate is converted into glyoxylic acid in a reactional liquid to accumulate glyoxylic acid and the reaction product is collected to efficiently provide the objective glyoxylic acid used as a raw material for producing vanillin and ethyl vanillin, production of ion exchange resin, an acid catalyst, etc., in pharmaceutical industry under mild conditions without producing a by-product.