Monitor-guide-tube fixing sections are fixed to portions on a core support plate side of a plurality of in-core monitor guide tubes disposed in a reactor vessel. The monitor-guide-tube fixing sections are fixed to the core support plate disposed in the reactor vessel to fix the in-core monitor guide tubes to the core support plate. This makes it possible to realize improvement of rigidity of the fixed sections of the in-core monitor guide tubes. A tie plate is fixed to the in-core monitor guide tubes. The in-core monitor guide tubes are coupled by the tie plate. This makes it possible to realize improvement of rigidity of the entire in-core monitor guide tubes. As a result, it is possible to realize vibration damping for the in-core monitor guide tubes.