PURPOSE: To rapidly dry the treated lumber by sucking air into a treatment chamber and causing an air flow around lumber and removing excess treatment liquid from lumber and discharging this excess treatment liquid together with the air from the treatment chamber and collecting it.

CONSTITUTION: An inlet part 32 and an outlet part 34 for lumber are provided to the side wall of a treatment chamber 30. Further the discharge openings 52b, 54b for discharging the air in the treatment chamber 30 are arranged to the positions higher than the inlet part 32 and the outlet part 34. Moreover a sucking and exhausting means 60 sucks air into the treatment chamber 30 from the gaps between the inlet part 32, the outlet part and lumber passed therethrough. The air flow is caused around lumber and discharged from an opening for discharge. A storage tank 56 is connected to this opening and arranged to collect the excess treatment liquid discharged together with the air by this sucking and exhausting means 60. As a result, the treatment liquid is saved and also drying time of the lumber after treatment is shortened and furthermore clean work is performed.