To provide a reinforcing metal capable of responding to opening of a building corner part in a building with staircase or stairwell by a wood-frame construction method to the outside of the building, or the deformation at an obtuse angle.

A conventional metal strap is bent in an L-shape along the building corner part and attached thereto. An L-shaped flat iron plate is welded orthogonally onto the center line of an iron plate bent in L-shape, whereby the angle of the bent iron plate is fixed. This is attached to a corner part having a floor opening part, whereby the corner part is prevented from being opened to the outside, and the strength performance of horizontal load bearing capacity of a floor structure plane can be safely, surely and easily improved.