PURPOSE: To enable to supply fluid with an increased pump head or flow rate as the case demands, by employing a combination of a first valve means interposed between the suction side of one pump and the discharge side of another pump and a second valve means interposed between the first valve means and a place to which the fluid is to be supplied.

CONSTITUTION: When the reactor pressure is lowered through condensation of steam in a pressure vessel 2 of a nuclear reactor while supplying cooling water to the pressure vessel 2 under a high pump head through series operation of a plurality of pumps arranged in two systems, globe valves 20, 23 constituting a first ON-OFF valve means are closed while globe valves 21, 22 constituting a second ON-OFF valve means are opened as shown in the drawing. Resultantly, pumps 5, 6 and pumps 7, 8 that have been operated respectively in series to each other are all operated independently from each other, so that an extremely large amount of cooling water can be supplied to the pressure vessel 2 of the nuclear reactor from a condensate storing tank 4 or a suppression pool 3 having four independent water supply sources. By employing such an arrangement, it is enabled to eliminate additional equipment such as high-pressure reactor core spray pumps that have been required in the conventional arrangements.