To ensure a required minimum floating force by constructing a floating body of two parts, one of which has a specified pressure resistant strength while the other has a spefified pressure resistant strength stronger than the former.

The volume ratio between a resin and glass beads of a floating body with a low pressure resistant strength is obtained. Then, a specific gravity at the time when the hollow glass beads with a low pressure resistance are beroken by pressure and water enters gaps in the beads is obtained. In assembling a floating body by the combination of floating bodies having two kinds of different pressure resistant strength, a limit level for preventing the whole floating body from sinking even if one of the floating bodies is broken is obtained. In the floating body by the combination thereof, a cylindrical substance 2 with a high pressure resistance is used as a floating body while a floating body 3 with a low pressure resistance is arranged to surround the substance 2. Accordingly, even if part of the whole floating body is damaged, it does not lose its floating force completely. As a result, it can be used for carrying necessary observation document or the like, and can be recovered.