PURPOSE: To use building timber for boxed timber used for columns, beams, etc., suppress the opening and closing of a kerf applied to the boxed timber and to prevent the deformation influencing bad effects on strength and appearance.

CONSTITUTION: After the dry treatment of boxed timber 10 to which a kerf 12 is applied, a notched groove 14 is formed in an open section 12a of the kerf 12. A plate-like joint member 16 is fitted to the notched groove 14, and it is firmly fixed with adhesives, etc. By the constitution, the opening and closing of the kerf 12 is suppressed with the joint member 16, and the overall deformation is prevented.