Provided is a flexible buoy lightweight, excellent in workability, and excellent in durability preventing the buoy from being damaged by contact and collision with another object. An airtight hollow structure (2) is formed of a flexible membrane made of rubber or resin, in which a reinforcement layer (C) is buried. A flange opening (4) having a gas inlet is fixed to at least one position of the hollow structure (2). Buoyancy thereof can be controlled by using an internal pressure of a gas injected in the hollow structure (2).