PURPOSE: To reuse a filter aid at low cost and to save expenses by dehydrating the spent filter aid with a hot oil bath and heating the dehydrated filter aid to the proper temp. to be calcined.

CONSTITUTION: A filter aid dehydrated to about 80% water content is immersed in oil heated to about 180°C in a hot oil bath device 2. The heating temp. is properly set according to the quality of the filter aid, etc. Next, the filter aid contg. the remained water content, oil content and filter cake is transferred to a ceramic column 30 by a belt conveyor 4 and heated to about 600°C to calcine it. The heating temp. is properly set according to the oil content and the filter aid to be calcined. Since the supplied filter aid includes an oil content and the heated oil content burns of itself, the combustion efficiency to the quantity of heat given on heating is high. On burning, since the oil content itself liberates heat, the heat efficiency is high. Filter cake consisting of organic substances is carbonized by this process.