PURPOSE: To improve the generation of varieties in the preheating of a work to be baked, which is caused in a tunnel furnace so far since preheating gas flows mainly through only the upper part of the inside of a furnace due to a low internal furnace pressure and a big gradient of reduction of the furnace pressure in a preheating zone.

CONSTITUTION: A blower 11, blowing air into a preheating zone X, is attached to the inlet port 2 of a furnace body 1. When air is blown into the furnace by the blower 11 against the flow of furnace gas, the internal furnace pressure of the preheating zone X is increased and the gradient of a furnace pressure is mitigated. Further, preheating gas flows uniformly across the whole of the section of the preheating zone X substantially whereby all works to be baked can be preheated evenly. On the other hand, the flow of the preheating gas becomes slow whereby the fore end side and the rear end side of the works to be baked in the transfer direction thereof are heated sufficiently and the difference between preheating temperatures can be reduced. The temperature gradient of the preheating zone X can be controlled also by regulating the amount of blown air.