PURPOSE: To increase the removal efficiency of gaseous components, by using a fractionating column consisting of a rectification plates section and a packing column section.

CONSTITUTION: Rectification plates such as bubble-cap trays valve trays, perforated plates or the like are used. As a packing material, Raschig rings, Lessing rings, saddles or the like are used. Which of the two types of the column should the upper is optionally decided and fractionation may be performed batchwise or continuously. The height of the packing part is 5W50% of the sum of the height of the plate column part and the height of the packing column part. Especially, high purity trioxane is separated and recovered from a mixture containing trioxane as the main component having high b.p. components, benzene as the main component having low b.p. components and formaldehyde as a dissolved gas component.