To provide a design method for an external structutal approach that a reasonable external structural approach corresponding to an arrangement space in a site can be easily and rapidly selected.

A depth (D) and the longest lateral dislocation length (Y) necessary for the minimum requirements of the actual construction are separately determined in respective external structural approach patterns. These dimensions and the arrangement space are compared with one another to select the optimum pattern. In particular, in a direct and straight external structural approach pattern (11) in which a direct entry type gate (1), an I-shaped approach (3), and a directly connected entrance porch (6) are combined together, the depth is set at 2,100 mm, and the lateral dislocation distance is set at 500 mm. In a direct and lateral type external structural approach pattern (12) in which the direct entry type gate (1), an I-shaped approach (3), and a laterally connected entrance porch (7) are combined together, the depth (D) is set at 2,100 mm and the lateral dislocation distance (Y) is set at 1,000 mm.