PURPOSE: To aim at enhancing the anti-vibration characteristic of an internal pump, by supporting three rings having different diameters for supporting the low section of a motor casing, with two pivots to relax the concentration of inertia force acting upon the motor section, to the tapered section of the motor casing.

CONSTITUTION: Each of internal pumps 3 (one of which is shown) annularly arranged in the bottom section of a nuclear reactor vessel, is held such that three rings 9 through 11 having different diameters are supported by means of two pivots 12, 13 within a support device frame 8 which is composed of two support rods 6, and the inner ring 11 is attached to the lower section of a motor case 5. With this arrangement, it is possible to restrain inertia force acting upon a motor section 4 from concentrating to the tapered section of a motor casing 5, and therefore, the anti-vibration characteristic of the internal pump 3 may be enhanced.