PURPOSE: To reform precisely even if it is of square lumber having strong bending tendency, by a method wherein forced bending and steam supply are performed together, then the forced bending is bent greatly in the opposite direction to the first bending by the original forced bending and pressing is applied to the square lumber so that the same is bent to some extent in the opposite direction to the first bending.

CONSTITUTION: Bent square lumber A is held within a processing case body 10 under a state where a bent projected surface side is turned upward and both ends parts of the lumber A are supported by a supporting pieces 11 provided on both sides of a longitudinal direction of the processing case. Then steam S through a steam generator 15 is supplied to an air pipe 13, spouted through a spouting hole 14 and filled out into the processing case body 10. Then a hydraulic unit 22 is driven, a jack 20 is actuated and pressing is applied to the square lumber A until the same is bent excessively in the opposite direction to a direction of the lumber A which has been bent in the beginning. Then an intercepting member 21 is removed, and the square lumber A is turned upside down and set up again to the processing case body 10. Then the jack 20 is actuated, the pressing the applied to the lumber A in the direction reforming a warp and on the contrary the same is bent about 4-5mm. Then the pressing by the jack 20 is released and the square lumber A is taken out of the processing case body 10 for natural drying.