PURPOSE: To improve the durability by placing a thin sleeve on a reinforcing framework formed by engaging ends of plural bars with notches in the peripheral part of a ring, and filling the synthetic binder in the sleeve along the whole length thereof.

CONSTITUTION: Ends of plural bars are engaged with notches provided in the peripheral part of a ring and arranged into the annular structure so as to form a reinforcing framework 3. A thin sleeve 1 is placed on the reinforcing framework 3, and a core 19 is arranged inside of the framework 3. Continuously, a space formed by the sleeve 1, the reinforcing framework 3 and the core 19 is filled with concrete 2, and after the concrete is hardened, the core 19 is removed.