A reactor metering pipe fixing device fixes a reactor metering pipe disposed in a reactor to a cylindrical outer surface of a jet pump diffuser and reduces stress that may be induced in a weld zone in the reactor metering pipe. An outer holding member 41 is held on the cylindrical outer surface 18a of a jet pump diffuser 18 by a C-shaped holding member 30. The reactor metering pipe 19 is clamped from radial directions by the outer holding member 41 and inner holding members 42 and 43. Wedges 44 and 45 are wedged into a gap between the cylindrical outer surface 18a and the inner holding member 42 and a gap between the cylindrical outer surface 18a and the inner holding member 43 to fix the reactor metering pipe 19 firmly to the cylindrical outer surface 18a.