PURPOSE: 5-Hydroxymethyl-2-furfral is oxidized with nitrogen oxides in an organic solvent to produce title compound useful as an intermediate of drug, agricultural chemical, perfume and furan resin readily in high yield with industrial advantage.

CONSTITUTION: 5-Hydroxymethyl-2-furfral readily obtained by dehydration of hexose under heating in the presence of an acidic catalyst is oxidized with an nitrogen oxide as dinitrogen tetraoxide, dinitrogen trioxide, nitric acid, nitrous acid, preferably dinitrogen tetraoxide, in an organic solvent as dimethyl sulfoxide or dimethoxyethane, preferably with dimethyl solfixide then nitric acid, to convert the primary alcohol selectively into aldehyde. The reaction temperature is 60W180, preferably 80W120°C and the amount of the oxidant used is more than 1.0, preferably 1.2W2.5 moles per mole of the starting material.