A top head forming a top head of a reactor pressure vessel is provided with a plurality of control rod drive mechanism (CRDM) penetration pipes arranged in a square grid-like pattern. The CRDM penetration pipes are joined with one another by a tube support member disposed among them. The top head is placed on a tank and water is filled in an inner region of the tank and the top head. A travel apparatus moves a nozzle lifting/lowering apparatus to a predetermined location and a lifting/lowering mechanism is operated to lift a jet nozzle 1 above the tube support member in the neighboring four CRDM penetration pipes. An ejection outlet of the jet nozzle is directed to a weld between one of the CRDM penetration pipes and the top head 18, and a water jet is ejected to start WJP. A nozzle turning mechanism turns the jet nozzle, and WJP is executed sequentially to the four CRDM penetration pipes.