A method for purifying a gas inside a polymer film production furnace with the use of the purification catalyst is provided. A purification catalyst for a gas inside a polymer film production furnace, contains a mixed oxide composed of a manganese-based oxide containing manganese and potassium and having a cryptomelane structure, and copper oxide. A method for purifying a gas inside a polymer film production furnace, includes a step 1 of bringing hot air containing volatile and/or sublimable organic substances, generated during production of a polymer film by the polymer film production furnace, into contact with the catalyst provided inside or outside the furnace, at a temperature in the range of 200 to 350° C. to decompose the organic substances oxidatively, and a step 2 of refluxing all or a part of a resultant decomposition gas to the polymer film production furnace.