A structure of a horizontal type cylindrical double-shell tank mounted on a ship includes: an inner shell storing liquefied gas; and an outer shell forming a vacuum space between the inner shell and the outer shell. A pair of support units supporting the inner shell is disposed between the inner shell and the outer shell. Each support unit includes: a plurality of cylindrical elements arranged in a circumferential direction of the tank such that an axial direction of each of the cylindrical elements coincides with a radial direction of the tank; a plurality of inner members each holding an end portion of a corresponding one of the cylindrical elements at the inner shell side; and a plurality of outer members each holding an end portion of a corresponding one of the cylindrical elements at the outer shell side. Each of the cylindrical elements is made of glass fiber reinforced plastic.