To improve productivity and placement, absorb thermal expansion of reactor pressure vessel in operation without restricting and ensure stable aseismicity.

In between the outer surface of a reactor pressure vessel 1 and the inner surface of reactor shield wall 2 covering around it, a plurality of vibration damper mechanisms 10 are installed with specific intervals in the circumference direction in this device. In this case, the vibration damper mechanisms 10 are provided with a plurality of metal plate springs 17 layered in the radial direction of the reactor pressure vessel 1 and each of vibration damper mechanisms 10 forms a slope B constituted of the ratio of each displacement so that the displacement due to thermal expansion of the reactor pressure vessel 1 in radial direction and axial direction in the plane where the vessel side support part 22 fixed on the outer surface of the reactor pressure vessel 1 and the support part 20 co-working with each plate spring 17 face to each other becomes free. Each plate spring 17 is provided with a specific spring constant.