PURPOSE: To make a structure strong by stacking truncated pyramid-like blocks on the surfaces of which projecting and recess parts are formed, with their tapered parts facing downward, on a desired place for a groin work or a foot protection work.

CONSTITUTION: Blocks a1, a2, b1, b2 formed by concrete where natural stone like projecting and recess parts are formed on the surface thereof are arranged side by side and stacked, with the tapered parts facing downward on the bottom of a riber. At this time, in the case of a groin work, blocks are constructed substantially perpendicularly to the flow extending from the riverside to the center of a river 1, and in the case of a foot protection work, the blocks are constructed along the riverside 2. If the blocks a1, a2, b1, b2 are stacked, the frictional force between the blocks is increased so as to increase the resisting force to the water flow. Furthermore, suitable voids are formed among the blocks so that an ecosystem good for circulating water or air is maintained. Locking metal fittings are buried in the respective blocks as required, and connected to each other by a chain or the like to surely fix the blocks. Thus, appearance can be improved and blocks can be easily and firmly constructed.