Abstract: A reactor feedwater system of a boiling water reactor includes: a reactor feedwater pump and a high pressure feedwater heater, that are arranged at an outside of a reactor containment vessel containing a reactor pressure vessel of a boiling water reactor, for pressurizing and heating a coolant; a main feedwater pipe for supplying the coolant, that are pressurized and heated by the reactor feedwater pump and the high pressure feedwater heater, to a side of the reactor containment vessel; and a plurality of branch pipes, that are connected to the main feedwater pipe, for pouring the coolant into the reactor pressure vessel. The main feedwater pipe is provided to the outside of the reactor containment vessel, and branching positions at which the branch pipes are branched from the main feedwater pipe are set to the outside of the reactor containment vessel, so that only the branch pipes penetrate through the reactor containment vessel and are connected to the reactor pressure vessel.