PURPOSE: To enhance natural recirculation power by connecting the extensible pipe extended into reactor pressure vessel to the connection part to a downcomer of a water supply pipe and directly and forcibly allowing a cooling material to flow downwardly in the downcomer.

CONSTITUTION: In this boiling water reactor, an extension pipe 10 having a length suspended in a downcomer 7 from the outlet of a water supply pipe 5 to reach a core 2 is connected to the outlet of the water supply pipe 5. Therefore, forcible flow force is applied up to the terminal opening part of the vertical downward extension pipe 10. Natural recirculation power such that a fluid lump having a sub-cool degree is held up to a relatively lower part and natural recirculation power is easy to apply in a local place is enhanced. Further, so called bypass phenomenon such that no downward stream is generated between the water supply pipe 10 and other water supply pipe present at the same level and inflow/outflow is generated between the water supply pipes can be prevented.