PURPOSE: To uniformly treat timbers to be treated which is sticker-piled in an independent sealed timber-treating furnace by controlling a flow rate of smoke hot-gas into the furnace according to an opening/closing amount of a control damper.

CONSTITUTION: Smoke hot gas generated from a smoke generating furnace 1 is forcibly fed through a duct 4 via a smoke hot gas feeding fan, and brought into uniform contact with timbers W to be treated sticker piled on a truck 9 in a timber treating furnace 5. An optimum temperature treating condition with the gas in the furnace 5 of the timbers W is previously recorded in a controller 15. The furnace temperature is decided based on the condition. The furnace temperature is detected by an in-furnace temperature detector 14. Its temperature is given as a signal to the controller 15. A opening/closing amount of a smoke hot gas control damper 6 is regulated according to an output signal from the controller 15.