PURPOSE: To make it possible to satisfactorily and smoothly carry out a coating process by jetting air from an auxiliary air feed pipe, corresponding to the sediment part of a granular material which is produced in a tank under the action of air feed from an air feed chamber through a filter, so that the granular material is uniformly floated.

CONSTITUTION: In a tank section 3, there are provided small pipe-like auxiliary air feed pipes 7 which are arranged being curved or crossed with a predetermined extent in a horizontal cross-section of the inside of the tank section 3. Further, air feed holes 9 are formed in the peripheral wall of each auxiliary air feed pipe 7 so that compressed air is fed from the air feed holes 9 into the tank section 3. Therefore, even though a sediment section of a granular material is formed due to compressed air blowing up from an air feed chamber 4 through a filter 2, since compressed air is fed from the air feed holes 9 in the auxiliary feed pipes 7, the sediment section falls into a floating condition. Thus, the granular material is uniformly floated, thereby it is possible to carry out a satisfactory and smooth coating process.