To provide a fish nest block for a river securing good environment for fish to live and facilitating regeneration even if the river bottom is flattened by concrete construction work.

In the fish nest block installed at the river bottom in parallel with a stream from the upper reaches to the lower reaches of the river, a slant face plate inclining downward in an upstream direction is connected to the upstream end of a top plate, and a leg piece is hangingly provided at the width center over both the top plate and slant face plate to provide habitable spaces of fishes on both sides of the leg piece. The lower end faces of the leg part piece and slant face plate are formed in the same or almost the same height positions so that the lower end of the leg piece and the lower end of the slant face plate serve as the ground plane.