PURPOSE: To improve treatment efficiency through the increasing of a temperature difference between a high temperature and a low temperature side by dividing the interior of a furnace into a plurality of blocks with a freely operating bulkhead doors and providing a subconveyor line which passes through only the block near an exit at a comparatively lower temperature.

CONSTITUTION: The interior of a drying furnace 1 is divided into, e.g. three blocks with two bulkhead doors 2, with a block near an entrance 1a at high temperature, a holding block in the middle which maintains a high temperature level and a block near an exit at low temperature. This drying furnace 1 is provided with a main conveyor line 3 running from one end to the other, and also a subconveyor line 4 passing through only a block 1c on the exit side after entering from the side of the furnace 1. Consequently, specific paint such as fluorine paint is baked at high temperature through the conveyor line 3, and paint which should be dried at comparatively lower temperature is dried by the subconveyor line 4.