To improve heat resistance, chemical resistance, water repellency, low frictional resistance and dust exfoliative property without spoiling filtration performance.

In a surface treatment method for a filter, the filter is impregnated with a dispersed liquid in which particulates of a fluororesin such as polytetrafluoroethylene and polychlorotrifluoroethylene and particulates of a polyimide resin such as poly(N,N'-p-phenylene-pyromellitic acid imide), poly (N,N'-p-phenylene-benzophenone tetracarboxylic acid imide) and poly (N,N'-p-phenylene-trimellitic acid amide imide) are blended in the ratio of 100: (5 to 70) (weight ratio) by a spray coating method or a roller coating method or a dipping method and then the filter is heated at 150 to 250°C.