An axial-flow turbine comprising: a turbine blade chamber in which a working fluid flow is formed; outer diaphragms (5, 8) which are consecutively installed a plurality of times along the working fluid flow to form an outer wall surface of the turbine blade chamber; turbine stages including stationary blades (3, 10) disposed on an outer diaphragm and buckets (2, 11) adjacent to the respective stationary blades and fixed to a rotor; and an extraction chamber provided on the outer circumference of the turbine blade chamber, the extraction chamber communicating with the turbine blade chamber through an extraction opening formed between the outer diaphragms (5, 8) consecutively installed along the working fluid flow, and having a downstream-side wall surface formed by the outer diaphragm; wherein the outer diaphragm forming the downstream-side wall surface of the extraction chamber is provided with a projection, the projection being formed more radially inwardly than the downstream-side edge on the outer circumference of the adjacent bucket on the upstream side of the extraction opening to form the downstream-side wall surface of the extraction opening, wherein the outer wall surface of the projection forms an upstream-side wall surface of the outer diaphragm for leading a part of the working fluid to the extraction chamber, and the inner wall surface of the projection forms an inner wall surface of the outer diaphragm for leading the remaining working fluid to the bucket on the downstream side of the extraction opening, whereby the upstream-side wall surface of the outer diaphragm is formed such that when an angle formed between a wall surface of the outer diaphragm facing the working fluid and an turbine central axis is referred to as a spread angle, the spread angle of the upstream side wall surface of the outer diaphragm -gradually increases with increasing distance from the upstream-side edge toward the downstream-side so as to gradually orient the working fluid flow outwardly as the working fluid flow advances toward inside of the extraction chamber, characterized in that the inner wall surface of the outer diaphragm is formed such that the spread angle thereof at the upstream-side edge is smaller than an average spread angle for a range from the upstream-side edge to downstream-side edge, and the spread angle thereof at the downstream-side edge is equal to an entrance spread angle of the outer edge of an adjacent bucket on the downstream side. a turbine blade chamber in which a working fluid flow is formed; outer diaphragms (5, 8) which are consecutively installed a plurality of times along the working fluid flow to form an outer wall surface of the turbine blade chamber; turbine stages including stationary blades (3, 10) disposed on an outer diaphragm and buckets (2, 11) adjacent to the respective stationary blades and fixed to a rotor; and an extraction chamber provided on the outer circumference of the turbine blade chamber, the extraction chamber communicating with the turbine blade chamber through an extraction opening formed between the outer diaphragms (5, 8) consecutively installed along the working fluid flow, and having a downstream-side wall surface formed by the outer diaphragm; wherein the outer diaphragm forming the downstream-side wall surface of the extraction chamber is provided with a projection, the projection being formed more radially inwardly than the downstream-side edge on the outer circumference of the adjacent bucket on the upstream side of the extraction opening to form the downstream-side wall surface of the extraction opening, wherein the outer wall surface of the projection forms an upstream-side wall surface of the outer diaphragm for leading a part of the working fluid to the extraction chamber, and the inner wall surface of the projection forms an inner wall surface of the outer diaphragm for leading the remaining working fluid to the bucket on the downstream side of the extraction opening, whereby the upstream-side wall surface of the outer diaphragm is formed such that when an angle formed between a wall surface of the outer diaphragm facing the working fluid and an turbine central axis is referred to as a spread angle, the spread angle of the upstream side wall surface of the outer diaphragm -gradually increases with increasing distance from the upstream-side edge toward the downstream-side so as to gradually orient the working fluid flow outwardly as the working fluid flow advances toward inside of the extraction chamber, characterized in that the inner wall surface of the outer diaphragm is formed such that the spread angle thereof at the upstream-side edge is smaller than an average spread angle for a range from the upstream-side edge to downstream-side edge, and the spread angle thereof at the downstream-side edge is equal to an entrance spread angle of the outer edge of an adjacent bucket on the downstream side.