Abstract: An improved device for effectively removing radioactive particles in moist gas has a pipe for feeding therethrough a moist gas containing radioactive solid particles, a solid-gas separation filter disposed in the pipe, a microwave generator and a wave guide connected between the pipe and the generator. Within the wave guide there is disposed a shielding plate capable of passing the microwave therethrough and preventing the spreading of contamination. Microwave from the generator is applied to the filter by means of the wave guide, and moisture condensate and the like on the filter is heated and evaporated by the microwave to minimize clogging of the filter.