PURPOSE: To improve the vesatility by forming a flow inlet and an overflow outlet in the upper parts of side walls on the opposite to each other, dividing a tank in two with a hindering plate which is set in parallel to the side walls and does not reach the bottom plate, and connecting rectangular tanks one another in the way the side walls in which a flow inlet and an overflow outlet are formed are attached closely, and filling the tanks with granular filtering materials, wherein the tanks are rectangular tanks whose bottom plates are formed like the bottom of a ship and which are made to be deepest immediately under the hindering plate.

CONSTITUTION: A tank main body 1 is formed into a rectangular shape and the upper end of side walls 2, 3 on the opposite each other are cut to form a flow inlet 7 and an overflow outlet 8. A hindering plate 9 is attached to another side wall 5 in parallel to the side walls 2, 3 and at the same time the lower end part of the hindering plate 9 is parted from the bottom plate 6. In this way, at least an upper half of the tank main body 1 is divided in two. The bottom plate 6 is so formed like the bottom of a ship as to be deepest immediately under the hindering plate 9 and ascend toward the flow inlet 7 and the overflow outlet 8. Tanks 1 prepared in this way are so connected with one another as to attach side walls 2, 3 closely and as to make a flow inlet 7 and an overflow outlet 8 communicate with each other. Each tank 1 is filled with a granular filtering material 18.