PURPOSE: To produce a pure soap by recycling waste oil without using a deleterious substance by mixing waste oil with a waste oil-treating agent in a vessel of a desired capacity under stirring, further adding water or hot water, allowed to stand for a certain time, then adding water or hot water again.

CONSTITUTION: An appropriate amount of waste oil and a powdery treating agent comprising an alkali metal salt of fatty acid, sodium orthosilicate and sodium sillicate in an amount of 70g per 100cc of the waste oil are charged in a vessel of desired capacity and they are stirred. Then, water or hot water at a certain temperature is added, they are stirred, then allowed to stand for a specific time and again water or hot water is added, and they are stirred. Thus, the waste oil such as domestic frying (TENPURA) oil can be recycled as creamy or soil soap simply without using the deleterious substance such as caustic soda.