PURPOSE: To enlarge a installation space of a structure, by hanging fixedly an outer barrel of a circulation pump on a flange underneath a penetration hole of a roof slab and by making an internal structure of the pump above a hydrodynamic structure of the pump and the outer supporting barrel, to be slender.

CONSTITUTION: A flange 6 having a spline notch in its inner side is placed underneath a penetration hole 2' of a roof slab 1 integrally with the slab. Prior to an installation of the slab 1 to a nuclear reactor vessel, an outer supporting barrel 4' of the pump to which a flange 8 having a projection on an outer circular surface is attached on the pump's upper end, is placed in correspondence to the notch of the flange 6. After placing the roof slab 1 on the nuclear reactor vessel, the flange 8 is inserted above the flange 6 of the penetration hole 2', the flange 8 is located above the flange 6 and then the flange 8 is rotated for the projection to be placed on the flange 6. A lock ring 11 filling a gap caused by the coincidence of the notch of the flange 6 and that of the flange 8, is inserted and the barrel 4' is hung fixedly on the flange 6. Subsequently, a pump internal 12 is inserted and a flange 13 is placed fixedly through inner sides of the lock ring 11 and others.