PURPOSE: To purify trioxane useful as a polyoxymethylene raw material in high purity, continuously and readily by bringing melt or a solution of trioxane into contact with a specific ionic adsorbent.

CONSTITUTION: Melt or a solution of trioxane is brought into contact with (A) an ionic adsorbent consisting essentially of at least two oxides selected from an alkali metal oxide, an alkaline earth metal oxide and oxides of a trivalent and a tetravalent element and an ionic adsorbent shown by the formula (M1 is at least one bivalent metal selected from alkaline earth metal, M2 is trivalent metal; An- is n-valent anion; (x) is in 0≤x<0.5; (m) is 0 or positive integer) and purified.