PURPOSE: To completely incinerate lump objects without cutting or pulverizing by burning the lump objects placed in a smke duct by a burning gas flow, and recirculating the burnt gas produced as a result of the burning of said lump objects into the main cinerating furnace together with the blown air.

CONSTITUTION: An auxiliary incinerating chamber 3 is added to the side of a main incinerating furnace 1, and the burnt hot gas drawn from the free board section 1b of the main furnace 1 is supplied from an upstream side smoke duct 2a. Lump objects placed in the auxiliary chamber 3 are thus burnt, and the resultant hot gas is recirculated through a downstream side smoke duct 2b into the main furnace 1. Part of the secondary air is branched to the blown into the downstream side smoke duct 2b, and its ejector effect extends to the upstream side smoke duct 2a to cause the burnt hot gas to be drawn from the free board section 1b. The burnt gas originating from the lump objects is mixed with the secondary air in the downstream side smoke duct 2b, and flows into the main furnace 1. The residue of the burnt lump objects in the auxiliary chamber 3 is thrown into the main furnace 1 together with ordinary waste materials for complete burning.