To form a reinforced concrete wall by means of a simple means, without necessitating construction of a form made of wood or assembling/ disassembling of a large-scaled scaffold.

A supporting member 4 is erected between a ring member 2, which is supported horizontally by a plurality of columns 1, and an annular foundation 3 having a large diameter which is arranged coaxially with the ring member 2. A bag member 5 supported by the supporting member 4 is expanded in order to cast a reinforced concrete wall 6 on the upper surface of the bag member 5. With regard to the internal space of the ring member 2 as well, the bag member 5a is expanded in order to cast a reinforced concrete wall 7 on the upper surface.