A double-shell ship tank (2A, 2B, 2C, 2D) comprising: an inner shell including an inner shell main part for storing a liquefied gas and an inner shell dome protruding upward from the inner shell main part; an outer shell including an outer shell main part surrounding the inner shell main part and an outer shell dome surrounding the inner shell dome; and at least three support mechanisms disposed around the inner shell dome between the inner shell and the outer shell, wherein each of the support mechanisms includes: a first support member fixed to one of the inner shell and the outer shell, the first support member including a first supporting surface parallel to a reference plane and a central axis of the inner shell dome that is included in the reference plane; a second support member fixed to the other one of the inner shell and the outer shell, the second support member including a second supporting surface facing the first supporting surface; and an insulating member interposed between the first supporting surface and the second supporting surface, the insulating member being fixed to the second supporting surface and configured to slide along the first supporting surface, and wherein the one of the first support member and the second support member that is fixed to the inner shell is positioned on the reference plane. an inner shell including an inner shell main part for storing a liquefied gas and an inner shell dome protruding upward from the inner shell main part; an outer shell including an outer shell main part surrounding the inner shell main part and an outer shell dome surrounding the inner shell dome; and at least three support mechanisms disposed around the inner shell dome between the inner shell and the outer shell, wherein each of the support mechanisms includes: a first support member fixed to one of the inner shell and the outer shell, the first support member including a first supporting surface parallel to a reference plane and a central axis of the inner shell dome that is included in the reference plane; a second support member fixed to the other one of the inner shell and the outer shell, the second support member including a second supporting surface facing the first supporting surface; and an insulating member interposed between the first supporting surface and the second supporting surface, the insulating member being fixed to the second supporting surface and configured to slide along the first supporting surface, and a first support member fixed to one of the inner shell and the outer shell, the first support member including a first supporting surface parallel to a reference plane and a central axis of the inner shell dome that is included in the reference plane; a second support member fixed to the other one of the inner shell and the outer shell, the second support member including a second supporting surface facing the first supporting surface; and an insulating member interposed between the first supporting surface and the second supporting surface, the insulating member being fixed to the second supporting surface and configured to slide along the first supporting surface, and wherein the one of the first support member and the second support member that is fixed to the inner shell is positioned on the reference plane.