To create the habitat of an underwater organism by forming an outer contour part by continuing a plurality of hollow leg parts formed by a porous iron plate on a hollow core part, and placing concrete on the outer contour part.

A disk-like cover part, a hollow side part cutting the upper part of a cone and a cylindrical connection part are fixed by welding or the like to form a leg part 1. Four connection faces providing a circular connection hole, a connection face connecting each connection face and four corner faces formed by a porous iron plate are fixed by welding or the like to form a core part 2. One end of a connection part fixed on the leg part 1 is inserted in the connection hole of the connection face of the core part 2 to be fixed by welding or the like to connect the leg part 1 and the core part 2 so as to form an outer contour part 10. Furthermore, concrete is placed from the opening part of the leg part 1 fixing no cover part of the outer contour part 10 to be solidified so as to form a block. Thereby the block or the like is set on a beach or the like, and the habitat of an underwater organism can be created.