An axial turbine includes a plurality of stages each comprising a plurality of stationary blades arranged in a row along the turbine circumferential direction and a plurality of moving blades in a row parallel to the stationary blades, each of the moving blade being disposed downstream of a respective one of the corresponding stationary blade in a flow direction of a working fluid so as to be opposed to the corresponding stationary blade. Herein, each of the stationary blades is formed so that the intersection line between the outer peripheral portion of the stationary blade constituting a stage having moving blades longer than moving blades in a preceding stage and a plane containing the central axis of the turbine, has a flow path constant diameter portion that includes at least an outlet outer peripheral portion of the stationary blade and that is parallel to the turbine central axis.