A steam turbine comprising: a rotor shaft that is configured to rotate about an axis; a plurality of rotor blade rows that are fixed to an outer side of the rotor shaft and disposed at intervals in an axial direction along which the axis extends; a casing that covers the rotor shaft and the plurality of rotor blade rows; and stator vane rows that are fixed to the casing, wherein each of the stator vane rows is disposed at intervals axially upstream of a respective rotor blade row of the plurality of rotor blade rows, wherein the casing has a diffuser that is configured to guide steam flowing out from a rotor blade row of a final stage, that is disposed axially most downstream among the plurality of rotor blade rows, to an outside of the casing, the diffuser including an outer guide that gradually expands radially outwards from its upstream end to its downstream end, an inner guide that is disposed radially inwards of the outer guide and gradually expands radially outwards from its upstream end to its downstream end, wherein an annular flow passage is defined between the outer guide and the inner guide, and characterized in that: the diffuser includes a first region that is a region closest to the rotor blade row of the final stage in the axial direction in which a cross-sectional area of the flow passage defined between the outer guide and the inner guide gradually decreases from one end to the other end in an axial downstream direction, and a second region that is connected to the first region on the downstream side in the axial direction, in which the cross-sectional area of the flow passage gradually increases from one end to the other end in an axial downstream direction. a rotor shaft that is configured to rotate about an axis; a plurality of rotor blade rows that are fixed to an outer side of the rotor shaft and disposed at intervals in an axial direction along which the axis extends; a casing that covers the rotor shaft and the plurality of rotor blade rows; and stator vane rows that are fixed to the casing, wherein each of the stator vane rows is disposed at intervals axially upstream of a respective rotor blade row of the plurality of rotor blade rows, wherein the casing has a diffuser that is configured to guide steam flowing out from a rotor blade row of a final stage, that is disposed axially most downstream among the plurality of rotor blade rows, to an outside of the casing, the diffuser including an outer guide that gradually expands radially outwards from its upstream end to its downstream end, an inner guide that is disposed radially inwards of the outer guide and gradually expands radially outwards from its upstream end to its downstream end, wherein an annular flow passage is defined between the outer guide and the inner guide, and characterized in that: the diffuser includes a first region that is a region closest to the rotor blade row of the final stage in the axial direction in which a cross-sectional area of the flow passage defined between the outer guide and the inner guide gradually decreases from one end to the other end in an axial downstream direction, and a second region that is connected to the first region on the downstream side in the axial direction, in which the cross-sectional area of the flow passage gradually increases from one end to the other end in an axial downstream direction. a first region that is a region closest to the rotor blade row of the final stage in the axial direction in which a cross-sectional area of the flow passage defined between the outer guide and the inner guide gradually decreases from one end to the other end in an axial downstream direction, and a second region that is connected to the first region on the downstream side in the axial direction, in which the cross-sectional area of the flow passage gradually increases from one end to the other end in an axial downstream direction.