PROBLEM TO BE SOLVED: To provide an air purification device that allows an unskilled person to easily and accurately determine, at any time, the replacement timing of a filter filled with a filter medium decomposing and removing harmful substances, such as formalin, by oxidation, and to provide a filter used in the air purification device.SOLUTION: A filter medium, in which an oxidizing agent decomposing and removing harmful substances, such as formalin, by oxidation and a reaction indicator whose color changes in accordance with an increase in oxidation products in the oxidizing agent are mixed, is charged into a filter casing 9 of a filter 7 provided with a first transparent part 9A in at least a part of a side wall facing an inner wall surface of a filter chamber 4 of an air purification device. At least a part of the filter chamber 4 is provided with a second transparent part 19A that allows observation of the filter medium inside the filter casing 9 through the first transparent part 9A from the outside of the filter chamber 4.
