PROBLEM TO BE SOLVED: To provide a method and a device for decomposing 1,4-dioxane, in each of which 1,4-dioxane can be decomposed with the reductivity and anaerobicity close to those in a subsurface environment without using a harmful oxidizer or the like.SOLUTION: The method for decomposing 1,4-dioxane comprises a step of making a reductant act on 1,4-dioxane in such a state that the i,4-dioxane in soil or ground water is brought into contact with a metal.