Disclosed herein is a floating structure with a fuel gas tank storing a gaseous fuel used as fuel in a dual fuel propulsion system disposed under a cargo space. According to the present invention, there is provided a floating structure with a fuel gas tank storing a gaseous fuel used as fuel and used while floating at sea, including: a fuel gas tank disposed in the hull of the floating structure; and a cargo space disposed on the top of the fuel gas tank on the upper deck of the floating structure.