To provide a U-shaped ditch which prevents water stagnation in a wiring slot in a cable conduit, to thereby suppress aged deterioration of a cable.

The U-shaped ditch 10 has formed therein the slot 11 in which a cable protective pipe is mounted. A lower wall surface of the slot 11 is inclined downward as it is closer to an opening, and therefore water stagnation in the slot is prevented. Further the slot 11 has a slipout preventive protrusion p for preventing slipout of the cable protective pipe therefrom. To eliminate defective filling of a material into a form or a mold, such as the case of placement of concrete, a protrusion forming member 12 having the shape of the slipout preventive protrusion p is prefabricated, and the protrusion forming member 12 is arranged at a predetermined location of a form for forming the U-shaped ditch. Then in this state, the material such as concrete is filled in the form, and therefore the protrusion forming member 12 and the filled material are connected to each other in one body.