PURPOSE: To obtain in an energy saving way trioxane, a monomer for polyacetal resin as an engineering plastics, by heating an aqueous formaldehyde solution in the presence of an acid catalyst followed by e.g. extracting the resultant trioxane with nitrobenzene.

CONSTITUTION: An aqueous formaldehyde solution is heated in the presence of an acid catalyst, and the trioxane formed is extracted from the aqueous solution into a nitrobenzene-contg. liquid phase; or, a vapor composed of formaldehyde and water containing the trioxane produced as vapor is brought into contact with a nitrobenzene-contg. liquid phase to extract the trioxane into the liquid nitrobenzene, thus obtaining the aimed trioxane in high efficiency.