To absorb a mounting error between a tank thermal insulation box and a hull block of an LNG tank transportation ship.

A hollow coupler 21 having a vertically quadrilateral cross section for fixing a thermal insulation box of an LNG tank is fixed on a hull block 4 surface by means of welding at its bottom part. In the quadrilateral inner peripheral wall of the coupler 21, a quadrilateral nut is loosely fit and fixed on the hull block 4. A penetrating hole 21a into which a coupler rod 11 is inserted is drilled on the upper surface of the coupler 21. The penetrating hole 21a is enough size for preventing the quadrilateral nut from coming off. The quadrilateral nut is made to be in a size with which a corner portion of the nut abuts against the inner wall surface of the coupler 21 to prevent the rotation of the quadrilateral nut, when rotating the nut.