To help prevention of slipping-off, slippage and torsion of a column caused by deterioration and vibration by forming corners and sides by adjacent faces, and making a box shape forming fixing holes at each face.

An auxiliary instrument 1 forms corners and sides on adjacent faces and makes a box shape for forming fixing holes on each face. The auxiliary instrument 1 is matched on the intersection part of columns B, C combined by mortise A insertion, the fixing holes are used to insert a through bolt E in a through hole D, and the bolt E is fixed by a nut F. The fixing hole of the other face provided on the auxiliary instrument 1 is used to fix a belt member Ha by a bolt G and the nut F, next, the columns B, C are surrounded by a belt material Hb, and tightened by the bolt G and the nut F to be tightened. In the case of the column B not using the through bolt E, the fixing hole is used to be stopped by a lug screw bolt to be fixed. Thereby the prevention of the slipping-off, slippage and torsion of the column can be helped.