PURPOSE: To remove contaminated matter on the surface and in the small cavity of membrane at the same time, by removing chemicals solution after contacting and permeating the chemicals solution generating gaseous body by the decomposition inside and outside of contaminated membrane and generating the gaseous body cont acting decomposing agent of the chemicals solution.

CONSTITUTION: Water solution contained chemicals generating gaseous body by the decomposition itself, is contacted with the inside or outside of contaminated membrane and the above solution is permeated in contaminated matter and fine cavity of the membrane and then, the solution is discharged after that. Next, gas is generated in the contaminated matter and fine cavity of the membrane, by contacting decomposing agent decomposing chemicals in the solution permeated in the contaminated matter and fine cavity of the membrane with the inside and outside of the contaminated membrane and the contaminated matter on the membrane is easily exfoliated from the membrane surface and also, the contaminated matter in the small cavity is made possible to move. Chemicals able to generate gas decomposing the contaminated matter, is especially effective among the gas generating chemicals.