To provide a load reduction device capable of simply reducing a load of a structure to be installed.

A load reduction device includes: a floating body 2 for being held in water; and a floating body supporting portion 10 which comprises a plate seat 3 of a flange structure for being attached to a supporting portion of a load reduction target structure, an intermediate support part 4 elongated toward the underwater floating body 2 from one end of the plate seat 3 to support the floating body 2, and an underwater supporting portion 5 for holding the floating body 2 in water on the tip side of the intermediate support part 4. The plate seat 3 can be fixed to the supporting portion of the load reduction device structure, and can also be sandwiched between the supporting portion and a lower structure of a foundation structure or the like. The underwater supporting portion 5 can also be achieved by using a net-like member and a basket-like member.