PURPOSE: To increase concentration of trioxan in vapor and to improve production efficiency in evaporating an aqueous solution of formaldehyde in the presence of an acidic substance under heating by adding a saccharide compound.

CONSTITUTION: An aqueous solution of formaldehyde (preferably one having high concentration) is blended with preferably ≥0.01wt.%, especially ≥1.0wt.% saccharide compound (e.g. monosaccharide such as glucose, oligosaccharide such as sucrose, polysaccharide such as starch or inositol) in the presence of an acidic substance such as zeolite, silica alumina or H type cation exchange resin, evaporated under heating to synthesize trioxan. Addition of the saccharide compound advantageously not only increases concentration of trioxan in vapor but has inhibitory effects on precipitation of settings. Trioxan is useful as a monomer for producing a polyoxymethylene.