To provide a beam mounting structure of a guard fence that can be installed using the same fitting for a slope and a curve point.

A first bolt hole 22 is formed with lateral guide grooves 23 laterally, and stop holes 24 are formed on the vertical sides of each guide groove 23. A second bolt hole 32 is formed as a cross-shaped hole comprising a vertically long hole part 321 and a laterally long hole part 322, and protrusions 33 are formed at the edge 323 of the laterally long hole part 322. When a beam B is laterally adjusted to a post 1, the protrusions 33 are inserted in the guide grooves 23 of a base fitting 2 and moved along the guide grooves 23. When the beam B is vertically inclined with respect to the post 1, the protrusions 33 can be inserted into the stop holes 24 of the base fitting 2. The guard fence can thereby be efficiently and surely installed using the same fitting even on a slope or a curve point.