PURPOSE: To improve drainage property and appearance by providing multiple through holes in the longitudinal direction on the side faces of a pipe having a triangular cross section, slantly arranging multiple pipes along the back face of a retaining wall, and connecting the lower ends to a drain port.

CONSTITUTION: A pipe made of polyethylene and having a triangular cross section is formed, and multiple through holes are provided in the longitudinal direction on all or two faces to obtain a drain pipe 1. Multiple pipes 1 are closely stuck on the back face of a slope face protective layer 3, and they are slantly fitted in zigzags. Lower ends of the pipes 1 are connected to one end of a drain pipe 5, and the other end of the drain pipe 5 is arranged on the front side of the lower section of the protective layer 3. Rain water and spring water are fed into the pipes 1 via the through holes and discharged to the outside of the protective layer 3 from the pipe 5. The passage is V-shaped and the inclination is large, the flow velocity is large, the drainage efficiency can be improved, no drain port is exposed on the slope face, and the appearance can be improved.