PURPOSE: To easily assemble so as to remarkably shorten a manufacturing time, and dispense with materials such as sheathing or stiffner so as to curtail manufacturing cost.

CONSTITUTION: A panel inserting through part 2 is provided along one vertical side of an outer frame panel 1. The panel inserting through part 2 is provided with a slotted hole 2a in which the other outer frame panel 1 can be inserted through. Four sheets of the outer frame panels 1 are used, the other outer frame panels 1 are inserted through the slotted holes 2a of the respective outer frame panels 1 and fixed by screws, and assembled into a nearly square so as to constitute the outer frame A of a formwork for a storage pool. An inner frame panel 4 is formed into an L-shaped section having a square corner part. One piece side of the inner frame panel 4 is formed with screw inserting through holes in the lateral direction, and screws are fitted on the other piece side so as to insert into the screw inserting holes 5. Four sheets of the inner frame panels 4 are used, the screws of the other inner frame panels 4 are inserted through the screw inserting holes of the respective inner frame panels 4 and fixed, and hence they are assembled into a nearly square.