A maintenance/repair device for reactor internal structure formed of a control-rod drive mechanism housing having a through-hole, for maintaining/repairing a reactor internal structure installed on a bottom of a reactor pressure vessel, the maintenance/repair device for reactor internal structure comprising: a device body configured to be fixed on an upper surface of the control-rod drive mechanism housing and having a seat base; and a welding-part repair mechanism connected to the device body, the welding-part repair mechanism being configured to repair a welding part of the control-rod drive mechanism housing; the device body comprising: a frame to which the welding-part repair mechanism is connected; and a guide connected to the frame and configured to be inserted into the through-hole formed on the upper surface of the control-rod drive mechanism housing and to be fitted to the inside of the control-rod drive mechanism housing for insertion into the through-hole; characterized by a clamp mechanism disposed in the guide and configured to fix the device body on the control-rod drive mechanism housing, wherein the clamp mechanism includes an air cylinder, a rod connected to the air cylinder, a rack connected to an end of the rod, a plurality of pinions that are rotatably disposed on the guide and have gears and pads that are disposed on the pinions and configured to project outward the guide so as to be pressed onto an inner surface of the control-rod drive mechanism housing when the device body is fixed on the control-rod drive mechanism housing, wherein the welding-part repair mechanism is configured to repair the welding part of the control-rod drive mechanism housing, while the seat base is seated on the upper surface of the control-rod drive mechanism housing and the pads are pressed onto the inner surface of the control-rod drive mechanism housing so that the device body is fixed on the control-rod drive mechanism housing. a device body configured to be fixed on an upper surface of the control-rod drive mechanism housing and having a seat base; and a welding-part repair mechanism connected to the device body, the welding-part repair mechanism being configured to repair a welding part of the control-rod drive mechanism housing; the device body comprising: a frame to which the welding-part repair mechanism is connected; and a guide connected to the frame and configured to be inserted into the through-hole formed on the upper surface of the control-rod drive mechanism housing and to be fitted to the inside of the control-rod drive mechanism housing for insertion into the through-hole; characterized by a clamp mechanism disposed in the guide and configured to fix the device body on the control-rod drive mechanism housing, wherein the clamp mechanism includes an air cylinder, a rod connected to the air cylinder, a rack connected to an end of the rod, a plurality of pinions that are rotatably disposed on the guide and have gears and pads that are disposed on the pinions and configured to project outward the guide so as to be pressed onto an inner surface of the control-rod drive mechanism housing when the device body is fixed on the control-rod drive mechanism housing, wherein the welding-part repair mechanism is configured to repair the welding part of the control-rod drive mechanism housing, while the seat base is seated on the upper surface of the control-rod drive mechanism housing and the pads are pressed onto the inner surface of the control-rod drive mechanism housing so that the device body is fixed on the control-rod drive mechanism housing. a frame to which the welding-part repair mechanism is connected; and a guide connected to the frame and configured to be inserted into the through-hole formed on the upper surface of the control-rod drive mechanism housing and to be fitted to the inside of the control-rod drive mechanism housing for insertion into the through-hole; characterized by a clamp mechanism disposed in the guide and configured to fix the device body on the control-rod drive mechanism housing, wherein the clamp mechanism includes an air cylinder, a rod connected to the air cylinder, a rack connected to an end of the rod, a plurality of pinions that are rotatably disposed on the guide and have gears and pads that are disposed on the pinions and configured to project outward the guide so as to be pressed onto an inner surface of the control-rod drive mechanism housing when the device body is fixed on the control-rod drive mechanism housing, wherein the welding-part repair mechanism is configured to repair the welding part of the control-rod drive mechanism housing, while the seat base is seated on the upper surface of the control-rod drive mechanism housing and the pads are pressed onto the inner surface of the control-rod drive mechanism housing so that the device body is fixed on the control-rod drive mechanism housing.