To prevent a side ditch main body and a side ditch cover from rattling as the result of twisting and to prevent the side ditch cover from being flipped up even if the load (unbalanced load) of a vehicle or the like from the outside is applied to one end of the side ditch cover.

An inclined face 3c having a rectilinear cross section is formed on the cover locking part 3 of a side ditch main body 1, and a lower inclined face having a smaller inclination angle θ2 than the inclination angle θ1 of the rectilinear inclined face 3c is formed at the corner of the lower surface of a side ditch cover 10, to form a contact corner part 11 making contact with the rectilinear inclined face 3c. At the corner part of the upper surface of the side ditch cover 10, an outermost end corner part 14 is formed on the outermost part where load (unbalanced load T) from the outside is applied, and the outermost end corner part 14 of the side ditch cover 10 is formed inward of the contact corner part 11.