To house a structural steel post at an accurate position by marking a post core or a return ink on a foundation of a base, aligning a hole core of a steel plate with the marking points of intersection, driving all anchors and mounting the steel plate horizontally at a specified position.

A hole core of a steel plate 3 is aligned with the point of marking intersection on a foundation of a base where all anchors 7 are driven, Then, the steel plate 3 is vertically sandwiched at a specified position with a unit 5 from the top of the foundation and mounted horizontally where the upper part of a collar of a bolt 1 is aligned with the height under the base of a pose with the upper nut 25 and a positioning jig is aligned with the marking ink and fastened with the power into 5. At that time, a center hole of the steel plate 3 is enlarged by about 15mm than the diameter of a screw of the bolt 1. A hole which allow the all anchor 7 to be placed is bored at a triangle part of the steel plate 3. This construction makes it possible to keep the structural steel post at an accurate position or at an accurate elevation.