A double-shell ship tank structure, which is a structure of a horizontal type cylindrical double-shell tank suitable to be mounted on a ship, the structure comprising: an inner shell storing a liquefied gas; an outer shell forming a vacuum space between the inner shell and the outer shell; and a pair of support units (4A, 4B) disposed between the inner shell and the outer shell and supporting the inner shell at positions spaced apart from each other in an axial direction of the tank; wherein each of the pair of support units (4A, 4B) 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, on an outer peripheral surface of the inner shell, an end portion of a corresponding one of the cylindrical elements at the inner shell side; and a plurality of outer members each holding, on an inner peripheral surface of the outer shell, an end portion of a corresponding one of the cylindrical elements at the outer shell side; each of the cylindrical elements being made of glass fiber reinforced plastic; and characterised in that the cylindrical elements and the inner members and the outer members which sandwich the cylindrical elements form a hollow load-transmitting structure extending in the circumferential direction of the tank, the cylindrical elements being spaced apart from each other by a distance smaller than a width of one cylindrical element. an inner shell storing a liquefied gas; an outer shell forming a vacuum space between the inner shell and the outer shell; and a pair of support units (4A, 4B) disposed between the inner shell and the outer shell and supporting the inner shell at positions spaced apart from each other in an axial direction of the tank; wherein each of the pair of support units (4A, 4B) 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, on an outer peripheral surface of the inner shell, an end portion of a corresponding one of the cylindrical elements at the inner shell side; and a plurality of outer members each holding, on an inner peripheral surface of the outer shell, an end portion of a corresponding one of the cylindrical elements at the outer shell side; 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, on an outer peripheral surface of the inner shell, an end portion of a corresponding one of the cylindrical elements at the inner shell side; and a plurality of outer members each holding, on an inner peripheral surface of the outer shell, an end portion of a corresponding one of the cylindrical elements at the outer shell side; each of the cylindrical elements being made of glass fiber reinforced plastic; and characterised in that the cylindrical elements and the inner members and the outer members which sandwich the cylindrical elements form a hollow load-transmitting structure extending in the circumferential direction of the tank, the cylindrical elements being spaced apart from each other by a distance smaller than a width of one cylindrical element.