To produce trioxane by using simple facilities.

This method for producing trioxane from an aqueous solution of formaldehyde comprises a step for bringing an aqueous solution having high concentration into a solid acid catalyst and a step for removing an acid. Specifically, the method for producing trioxane comprises each step for producing trioxane from formaldehyde by using organic sulfonic acids as the solid acid catalyst in a reactor 2 of figure 1, extracting the product by using a solvent which is insoluble to water in an extraction tower 3 and removing the acid by using an anion exchange resin in an acid removing tower 5.