PURPOSE: To simplify the upper reactor mechanism and make it easy to maintain a refueling device by eliminating the refueling device using a rotary plug to be replaced by the refueling device capable of operating in a reactor vessel.

CONSTITUTION: Underneath a shield plug 6 closing the upper opening of a reactor vessel 1, a plurality of rails 18 in reactor are placed and a refueling device 19 is run on these rails 18 in reactor. In the refueling device 19, grippers 21 and wires 20 to elevate fuel assemblies 10 are provided. In the shield plug 6, a penetration hole 17 for inserting the refueling device 19 into the reactor vessel 1 is provided. Above this penetration hole 17, a plug 16 for the refueling device is placed and a cask 22 is set on this plug 16. A refueling platform car 23 is connected with the cask 22.