As a pretreatment of a manufacturing process of sintered ore, a sintering material including iron ore, a SiO.sub.2-containing material, a limestone base powdery material and a solid fuel type powdery material is projected as a additional coating auxiliary raw material into the drum mixer with a additional coating conveyor disposed in proximity to an exhaust outlet of the drum mixer. Preferably, a sintering material excepting a limestone base powdery material and a solid fuel type powdery material is charged from a charge inlet of the drum mixer to granulate and in a region disposed on a downstream side, an additional coating auxiliary raw material including a limestone base powdery material and a solid fuel type powdery material is added, and thereby until reaching the exhaust outlet, the additional coating auxiliary raw material is deposited and formed on a exterior coating portion of the sintering material.