To ensure maintenace and restoration of ecosystem at low cost by making a bottom board of concrete block whose side wall having U shape in section is perpendicular or inclined to the outer side thicker than the thickess of the side wall, and dividing the concrete block into sections of recess part extending over all the surface of bottom and cut part.

A first PC block 21 is used whose bottom part wall thickness is the same as the side wall part and a second PC block 22 is used where a gravel 4 storing recess 2 is formed at the inside of the bottom part having a thicker all than the side wall part. A plurality of the first PC blocks 21 is aligned and therebetween the second PC block 22 is assembled and connected each other. A long waterway can be formed by using general first PC blocks inexpensively. The water flow controlled by the first PC blocks 21 is turned into an effective little steam state by means of the recess 2 filled with gravel. By always moving the filled gravel and the like under the flow of water the surface of air is renewed.