PURPOSE: To obtain a compound useful as a synthetic intermediate for hydantoins in high purity and yield while suppressing formation of by-products and preventing coloring, by adding prussic acid to a formalin solution containing dissolved sodium acetate, carrying out at a specific pH value and removing the prussic acid.

CONSTITUTION: Crystals or an aqueous solution of sodium acetate is added to formalin and dissolved therein to give a formalin solution and prussic acid kept at a temperature within the range of 15W30°C is then added thereto. Since the pH of the reaction solution varies with the addition of the prussic acid, acetic acid or sodium acetate is added, as necessary, to keep the pH of the reaction solution at 4.8W6.0 and carry out the reaction. The pH of the reaction solution is then adjusted to ≤2, preferably 1.5 with sulfuric acid to remove the residual prussic acid at ≤50°C under reduced pressure to afford the aimed substance.