PURPOSE: To prevent the formation of a gap or an uneven step between a shoulder and an L-shaped retaining wall L when the drain is formed along the L- shaped retaining wall L of a shoulder.

CONSTITUTION: A pair of side sections 12 and 12 are drooped to both sides of a top plate section 11, and the top plate section 11 is extended to one side section 12 to form a locking section 13. When the locking section 13 is so set that it is hooked to the upper end of a side wall section L2 of an L-shaped retaining wall L, leveling work is not required, and there is no possibility to form a gap or an uneven step, etc., between the L-shaped retaining wall L and a shoulder.