To provide a horizontal bar mounting structure for facilitating the mounting work and enabling adjustment of a mounting angle of the horizontal bar according to the inclination of the place of installation when the horizontal bar is fitted between posts.

A mounting metal piece 3 comprises a mounting plate part 31 fitted to the post 1 and a substantially L-shaped receiving plate part 32 supporting the horizontal bar 2. When the mounting metal piece 3 is mounted on each post 1, a rear face of a supporting plate part 31 is joined to a front side of one of the adjacent posts 1. In an opposite manner, the rear face of the supporting plate part 31 of another mounting metal piece 3 is joined to a rear side of the other post 1 so that end parts of the horizontal bar 2 are supported by the receiving plate parts 32 of the two mounting metal pieces 3.