combusting unit 3 is an air feed line 4 having a blower 2b for supplying air to the infusiblization waste gas in the combusting unit 3 air is mixed with the infusiblization waste gas and the combustible component of the activation waste gas and the resulting mixture is combusted the hot products of combustion in the combusting unit 3 are exhausted via a waste gas line 5 disposed downstream of the combusting unit 3 that is to say the low temperature side of the combusting stage is a preheating unit 6 provided with a preheating pipe the preheating pipe is supplied with an infusiblizing gas such as air through an infusiblizing gas feed line 7 equipped with a blower 2c this infusiblizing gas is preheated by the hot products of combustion from the combusting unit 3 and fed to the infusiblizing unit b through a feed line 8 a preheating unit 9 equipped with a preheating pipe is disposed upstream of the combusting unit 3 i e on the high temperature side of the combusting stage to the preheating pipe is connected an activating gas feed line 10 having a blower 2d for supplying steam as the activator the activating gas fed to the preheating pipe is preheated by the hot products of combustion of the combustible component from the combusting stage and fed to the activating.