PURPOSE: To prevent decrease of penetrating capacity, by providing a bottom cover provided with a number of holes on a penetrating bed at the bottom and forming a clearance between the bottom cover and the penetrating bed.

CONSTITUTION: Reinforced concrete blocks 2 composed of two opposite side plates 3 and connection beams 4 connecting the inner side plates 3 and unifying them, are buried in a waterway. A penetrating bed 7 like sand is laid on the bottom of the blocks 2 and a bottom cover 5 having a number of holes 6 is laid on the penetrating bed 7 to form a clearance 8 between the bottom cover 5 and the penetrating bed 7. And further, the bottom cover 5 is laid on the penetrating bed 7. In this way, sludge hardly reaches to the penetrating bed 7 and hence, the penetrating bed 7 is hardly clogged with sludge and decrease of penetrating capacity thereof is prevented.