In order to have the conveyor (chain) D2 bypass the pre-heating step C2, the conveyor D2 may be disposed in common with the conveyor D1 in such a manner that the bodies W are transferred to the conveyor D2 in the downstream of the pre-heating step C2 or C2-2 from the conveyor D1 in the upstream of the pre-heating step C2 or C2-2 by means of a transferring device such as lifters.