PURPOSE: To provide a method of deodorizing waste oil for pure soap and a method of producing pure soap which can effectively remove odor peculiar to waste oil such as animal or vegetable oil or their mixture, for example, waste 'Tempra' oil to be reused, in no need of caustic soda.

CONSTITUTION: An appropriate amount of waste animal oil, waste vegetable oil or waste oil mix thereof is heated with a heater over 180°C and stirred, as an appropriate amount of a deodorizing additive made of a vegetable acidulant is added, then spontaneously cooled down to room temperature, finally filtered through a prescribed mesh filter whereby the waste oil is deodorized. In addition, the deodorized waste oil is mixed with a waste oil-treating agent, stirred, further stirred together with an adequate amount of water or hot water, and finally allowed to stand for a prescribed time to give a starting oil of pure soap in a creamy form free from the waste oil odor.