To provide a fall prevention device and a tank for floating body structure reducing installation costs and improving the safety of workers. This fall prevention device includes: a rail member (2) that is installed along a passageway P adjacent to a wall surface (W) and is fixed to the wall surface (W); and a guide member (3) which is slidably disposed on the rail member (2) and to which a life-line (L) latches. The rail member (2) also serves as a handrail for movement. The rail member (2) has a by-pass part (21) that in a case where a plurality of guide members (3L and 3R) face each other causes one guide member (3R) to be bypassed. The by-pass part (21) is configured by, for example, a by-pass channel branching off from one part of the rail member (2) and connecting to another part of the rail member (2), or a dead end that branches off from a part of the rail member (2).