A method for repairing a reactor vessel covered with cooling water, the method comprising: hanging down a tubular frame of which an interior is in an air environment having a bottom board into the cooling water; setting a lower portion of said tubular frame in said reactor vessel while said interior of the tubular frame having said bottom board is in said air environment; and repairing said reactor vessel, wherein said frame is formed by a platform (13, 14) set at an interior of said reactor vessel and cylindrical containers connected and laminated on said platform (13, 14), said platform (13, 14) is formed by a substantially cylindrical shaped sidewall (13a, 14a) of which an outer diameter is smaller than an inner diameter of said reactor vessel and a bottom board for closing a lower end of said sidewall (13a, 14a), wherein an access window for connecting a tubular frame (21, 22) provided at a side wall of said reactor vessel is formed at said sidewall (13a, 14a) and said access window can be sealed by attaching a cover body, said platform (13, 14) comprises a flange portion protruded from an upper end of said sidewall (13a, 14a) toward an outer peripheral direction and supported by an upper end of said reactor vessel and a sealing element is inserted to a gap between said upper end of said reactor vessel and said flange portion, an underwater fastening portion for tightly fastening said flange portion with respect to said upper end of said reactor vessel is provided, wherein said underwater fastening portion comprises: a pin upwardly fixed on and perpendicularly standing with respect to said upper end of the reactor vessel, a wedge portion formed on an upper surface of the flange portion, a taper key slidably arranged on a taper surface of the wedge portion, a clamp cylinder of which one end is connected to the taper key and the opposite end is connected to the flange portion, a big hole portion formed by penetrating both the flange portion and the wedge portion, wherein the diameter of the big hole portion is larger than a head portion of the pin and said head portion of the pin penetrates the big hole portion, and a groove portion is formed at the taper key wherein the width of the groove portion is narrower than the width of the head portion of the pin and wider than a width of a main body of the pin. hanging down a tubular frame of which an interior is in an air environment having a bottom board into the cooling water; setting a lower portion of said tubular frame in said reactor vessel while said interior of the tubular frame having said bottom board is in said air environment; and repairing said reactor vessel, said frame is formed by a platform (13, 14) set at an interior of said reactor vessel and cylindrical containers connected and laminated on said platform (13, 14), said platform (13, 14) is formed by a substantially cylindrical shaped sidewall (13a, 14a) of which an outer diameter is smaller than an inner diameter of said reactor vessel and a bottom board for closing a lower end of said sidewall (13a, 14a), wherein an access window for connecting a tubular frame (21, 22) provided at a side wall of said reactor vessel is formed at said sidewall (13a, 14a) and said access window can be sealed by attaching a cover body, said platform (13, 14) comprises a flange portion protruded from an upper end of said sidewall (13a, 14a) toward an outer peripheral direction and supported by an upper end of said reactor vessel and a sealing element is inserted to a gap between said upper end of said reactor vessel and said flange portion, an underwater fastening portion for tightly fastening said flange portion with respect to said upper end of said reactor vessel is provided, wherein said underwater fastening portion comprises: a pin upwardly fixed on and perpendicularly standing with respect to said upper end of the reactor vessel, a wedge portion formed on an upper surface of the flange portion, a taper key slidably arranged on a taper surface of the wedge portion, a clamp cylinder of which one end is connected to the taper key and the opposite end is connected to the flange portion, a big hole portion formed by penetrating both the flange portion and the wedge portion, wherein the diameter of the big hole portion is larger than a head portion of the pin and said head portion of the pin penetrates the big hole portion, and a groove portion is formed at the taper key wherein the width of the groove portion is narrower than the width of the head portion of the pin and wider than a width of a main body of the pin. a pin upwardly fixed on and perpendicularly standing with respect to said upper end of the reactor vessel, a wedge portion formed on an upper surface of the flange portion, a taper key slidably arranged on a taper surface of the wedge portion, a clamp cylinder of which one end is connected to the taper key and the opposite end is connected to the flange portion, a big hole portion formed by penetrating both the flange portion and the wedge portion, wherein the diameter of the big hole portion is larger than a head portion of the pin and said head portion of the pin penetrates the big hole portion, and a groove portion is formed at the taper key wherein the width of the groove portion is narrower than the width of the head portion of the pin and wider than a width of a main body of the pin.