An axial flow rotating machine has: a rotor with a plurality of rotor blades; a stator with a plurality of stator blades; an axial flow rotating portion defined by the rotor and the stator; and a diffuser connected to the axial flow rotating portion on a downstream side of the axial flow rotating portion. A final blade portion inner-circumferential inner wall, which is a portion of an inner-circumferential inner wall of the axial flow rotating portion, is defined such that a diameter thereof at a trailing edge position of a final blade is smaller than the diameter at a leading edge position of the final blade. In addition, a diameter of all or a portion of a diffuser inner-circumferential inner wall decreases in a direction of the downstream side in an axial direction.