PROBLEM TO BE SOLVED: To improve soundness of a DP/LC pipe system under conditions where replacement of an in-core portion of the DP/LC pipe system is difficult.SOLUTION: A repair method of an ex-core double pipe in a repair method of a differential pressure detection/boric acid solution injection system pipe being provided so as to pass through a pressure vessel 13 and having an ex-core double pipe 39 outside the pressure vessel 13, an in-core double pipe 38 inside the pressure vessel 13 and a passing through portion passing through the pressure vessel 13 and being connected with the ex-core double pipe 39 and the in-core double pipe 38 includes: a cutting step of cutting and removing the existing ex-core double pipe 39; and a communication step of connecting a new ex-core double pipe 46 in lieu of the ex-core double pipe 39 removed to a cut portion formed by cutting the existing ex-core double pipe 39 and causing the new ex-core double pipe 46 to communicate with the passing through portion and the in-core
