Abstract: A boiling water reactor comprises a reactor pressure vessel accommodated in a vertical fashion, a core disposed at a low portion in the reactor pressure vessel, a plurality of control rods to be inserted from an upper side of the core into the spaces between adjacent fuel assemblies and withdrawn therefrom upwardly, a shroud surrounding the fuel assemblies so as to define the core and having an upper end opening, a shroud head which closes the upper end opening of the shroud and through which the control rods are inserted or withdrawn, a separator standing upward from the shroud head to carry out gas-water separation of steam generated from the core, a fixing pedestal disposed above the separator, a control rod driving mechanism mounted on the fixing pedestal and adapted to drive the control rods, a drier unit including a plurality of drier elements annularly arranged along an upper inner wall surface of the reactor pressure vessel and adapted to dry the steam passing through the steam flow hole of the fixing