In a method for preventing dust of a powdery material by mixing the powdery material with a polytetrafluoroethylene and subjecting the mixture to a compression-shearing action taken at temperatures from about 20 to 200°C thereby the polytetrafluoroethylene is fibrillated to prevent dusting of the powdery material, the method is characterized by that polytetrafluoroethylene is an aqueous emulsion of a homopolymer of tetrafluoroethylene, the homopolymer has a specific gravity of not larger than 2.20 and an average particle size of 0.10-0.50µ, and the emulsion contains a hydrocarbon-base anionic surfactant of not less than 1.0wt% of the polytetrafluoroethylene as the emulsion stabilizer. The emulsion is highly stable and a less possible contaminant of underground water.