To prevent the bending of a floating body due to a current stream or a leakage of air in the floating body by overlapping two flexible sheet materials, bonding their upper and lower end parts, providing an anchor locking tool at an adequate position in a lower end part of the floating body, and providing a bending prevention plate integrally at a predetermined interval.

Two flexible sheet materials 1, 2 are overlapped, and their upper and lower end parts are bonded mutually to form a cylindrical body. At this time, a bending prevention plate 8 is provided integrally across a predetermined interval between adjacent bending prevention plates 8 in a part where the upper and lower end parts of a floating body are bonded. Moreover, it is preferable that an anchor locking tool 6 for locking an auxiliary floating body and a weight is provided on the bending prevention plate 8. By providing the bending prevention plates 8 across a predetermined interval in this way, it is possible to prevent the bending and breakage of the floating body effectively even if the floating body is developed in a place where current stream is rapid.