To form a large-sized structure having a large area by inserting PC steel bars among U-shaped plate members containing the abutting sections of parting faces and flat plate members by combining each split and formed members and stretching and fixing the PC steel bars.

A frame body is divided into U-shaped plate members 2a, 2b on both sides and flat plate members 4a, 4b and precast-molded, and parting faces as abutting sections 5 are set at the point places of external pressure of zero, zero to moment curves. PC steel bars 6 are inserted to each section of each blade-piece plate section 3a, 3b and the flat plate members 4a, 4b respectively, and stretched and connected and fixed by nuts 9 in fixing grooves 7, 8, thus forming the frame body 1 for a pneumatic caisson. Laminated frame bodies superposed to the frame body 1 are unified by inserting a large number of the PC steel bars 6 at intervals and stretching, coupling and fastening the PC steel bars 6 by the nuts 9. A structure assembled in this manner is sunk under the ground from a cutting edge in the lower section of the frame body 1 for the pneumatic caisson, and an underground parking is manufactured.