Abstract: In a rotary drum type device for coating granular solids of the type having a rotary drum with a perforated cylindrical wall section, a supporting ring surrounding the cylindrical wall section of the drum for rotatably supporting the same, and inlet and outlet duct means defined within the supporting ring and opening to the perforated cylindrical wall section of the drum in such a way that the hot air or the like is directed from the top of the drum toward a tumbling bed of granular solids formed within the drum, the perforated cylindrical wall section is so divided that the area which faces the opening of the inlet duct means will not face the opening of the outlet duct means and vice versa, whereby the coating agent which is deposited on the walls of the small holes of the cylindrical wall section facing the opening or inlet of the outlet duct means can be prevented from being separated and falling into the drum by the hot air or the like flowing through the opening of the inlet duct means into the drum.