PURPOSE: To produce dried lumbers, whose qualities are uniform, highly efficiently with a high yield by providing the inside of a lumber receiving chamber with a high temperature atmosphere of uniform low oxygen concentration.

CONSTITUTION: When lumbers are carried into a lumber receiving chamber 1 to seal them and fuel in a combustion gas generating furnace 2 is ignited subsequently, combustion gas flows from the upper part of the combustion gas generating furnace 2 into the lumber receiving chamber 1 through a combustion gas supplying passage 3. When a damper 61 is closed in this case, air in said lumber receiving chamber 1 is sucked into the combustion gas generating furnace 2 through a gas recovering passage 5 as the lumber receiving chamber 1 is filled with high-temperature combustion gas from the upper part thereof, then, the lumber receiving chamber 1 is filled with the high-temperature combustion gas wholely. In this case, gas in the lumber receiving chamber 1 is circulated forcibly to the combustion gas generating furnace 2 through the gas recovering passage 5 by using a fan 51 and a damper 52 whereby air, supplied to the combustion gas generating furnace 2, is diluted by circulating gas to maintain an oxygen concentration at a low level and prevent the oxygen concentration in the lumber receiving chamber 1 from being fluctuated while the unification of a temperature distribution in the same chamber 1 can be promoted.