In a nuclear reactor internal structure, generation and promotion of object-downstream separating vortices are suppressed so that coolant uniformly flows in a reactor core and pressure loss of flow of the coolant is reduced so that the flow of the coolant is stabilized. A lower connecting plate 30 arranged in a lower plenum 8 comprises a ring portion 31 in which an arcuate portion 32 and a cut-off portion are alternately formed. An outer peripheral portion of the ring portion 31 is asymmetric relative to a flow direction of main flow and also asymmetric relative to a separating flow generation direction.