PROBLEM TO BE SOLVED: To provide a decomposing agent of chlorine dioxide and a decomposing method of gaseous chlorine dioxide, which are means to efficiently decompose the gaseous chlorine dioxide within a shorter time period.SOLUTION: At least one of manganese oxide and a permanganate is made to be contained in a decomposing agent of chlorine dioxide as an active ingredient. The decomposing agent of chlorine dioxide is not consumed in the decomposition reaction as it acts as a catalyst, so that labor and cost required in maintenance of a chlorine dioxide decomposing apparatus are suppressed. In addition, the decomposing agent can be included in a chlorine dioxide decomposing filter medium.
