An axial turbine, comprising: an upstream turbine stage including: a plurality of upstream nozzles arrayed in a tangential direction between an upstream diaphragm outer ring and an upstream diaphragm inner ring; a plurality of upstream buckets disposed on an outer peripheral side of a turbine rotor and arrayed in the tangential direction; and a cover disposed at a distal end of the upstream bucket and opposed to an inner wall of the upstream diaphragm outer ring across a gap; and a downstream turbine stage including: a downstream diaphragm outer ring disposed downstream of the upstream turbine stage, the downstream diaphragm outer ring having an inner peripheral-side end wall shaped into a flare; a plurality of downstream nozzles arrayed in the tangential direction between the downstream diaphragm outer ring and a downstream diaphragm inner ring; and a plurality of downstream buckets disposed on the outer peripheral side of the turbine rotor and arrayed in the tangential direction, the inner peripheral-side end wall of the downstream diaphragm outer ring having a flare angle formed to be greater than a slant angle of an inner peripheral-side wall of the cover, wherein the inner peripheral-side end wall of the downstream diaphragm outer ring has a meridional shape curved into an S-shape that has an inflection point such that a tangent at the inflection point with respect to a steam flow direction has a positive gradient, characterized in that the inflection point is between the upstream bucket and a front edge of the downstream nozzle. an upstream turbine stage including: a plurality of upstream nozzles arrayed in a tangential direction between an upstream diaphragm outer ring and an upstream diaphragm inner ring; a plurality of upstream buckets disposed on an outer peripheral side of a turbine rotor and arrayed in the tangential direction; and a cover disposed at a distal end of the upstream bucket and opposed to an inner wall of the upstream diaphragm outer ring across a gap; and a plurality of upstream nozzles arrayed in a tangential direction between an upstream diaphragm outer ring and an upstream diaphragm inner ring; a plurality of upstream buckets disposed on an outer peripheral side of a turbine rotor and arrayed in the tangential direction; and a cover disposed at a distal end of the upstream bucket and opposed to an inner wall of the upstream diaphragm outer ring across a gap; and a downstream turbine stage including: a downstream diaphragm outer ring disposed downstream of the upstream turbine stage, the downstream diaphragm outer ring having an inner peripheral-side end wall shaped into a flare; a plurality of downstream nozzles arrayed in the tangential direction between the downstream diaphragm outer ring and a downstream diaphragm inner ring; and a plurality of downstream buckets disposed on the outer peripheral side of the turbine rotor and arrayed in the tangential direction, a downstream diaphragm outer ring disposed downstream of the upstream turbine stage, the downstream diaphragm outer ring having an inner peripheral-side end wall shaped into a flare; a plurality of downstream nozzles arrayed in the tangential direction between the downstream diaphragm outer ring and a downstream diaphragm inner ring; and a plurality of downstream buckets disposed on the outer peripheral side of the turbine rotor and arrayed in the tangential direction, the inner peripheral-side end wall of the downstream diaphragm outer ring having a flare angle formed to be greater than a slant angle of an inner peripheral-side wall of the cover, wherein the inner peripheral-side end wall of the downstream diaphragm outer ring has a meridional shape curved into an S-shape that has an inflection point such that a tangent at the inflection point with respect to a steam flow direction has a positive gradient, characterized in that the inflection point is between the upstream bucket and a front edge of the downstream nozzle.