PURPOSE: To provide a purifying method of clay capable of completely filtering clay off by with filter and incorporating no clay into an end-product even if the clay is used for regenerating (purifying) a spent food oil (waste oil) and to provide its device.

CONSTITUTION: The subject method comprises the steps of (1) adding water to the clay under stirring; (2) filtering a suspension under stirring in the first step or just after stirring with a filter having at least 0.008-0.01mm mesh; (3) drying the clay after filtration in the second step; and (4) filtering the clay dried in the third step with vibration with a filter of 0.011-0.08mm mesh. Since the clay is treated to be a prescribed grain size, the clay is surely filtered off by using the filter even if the clay is used as a adborbent at the time of regenerating a spent food waste oil, and even a trace of clay does not remain in a food oil after regeneration therefore, odor due to burning of silicon oxide is not generated even when this food oil is heated.