A column base fitting 42 includes a bottom plate 42c having an approximately square plate shape and a support base 42f inside from the peripheral part of an upper surface 42d of the bottom plate 42c. A first bolt insertion holes 42a is formed in each four corner portions of the bottom plate 42c and a second bolt insertion hole 42b is formed at two positions closer to the center part than the first bolt insertion hole 42a, in the length direction of each four sides of the bottom plate 42c. A corner built-up portion 42r is formed at both ends in the length direction of the side surface 42h of the support base 42f, having a shape protruding outward in the perpendicular direction from the side surface 42h and filling the corner portion between the side surface 42h and an upper surface of the bottom plate 42c.