The problem to be solved in the present invention is to assemble an LNG tank on land and load the same on board the hold of a ship to shorten the construction time for building an LNG ship. In the present invention, a cold storage material and a membrane are stretched over the inside of a building frame container to make an LNG tank, which is then loaded on board the hold of a double hull structure. Beams are welded to the outer surface of the building frame container to provide adequate reinforcement prior to cold storage construction so as to prevent deformation of the building frame container during on-board loading. When the tank is being loaded on board the hull, the beams of the building frame container are connected to an inner shell of the hull to integrate the LNG tank and the hull, thus supporting a liquid-load weight in a shared manner between the building frame container and the hull.