A steam valve comprising: a casing with a steam flow path formed therein; a valve body disposed in the steam flow path and provided so as to be capable of moving relative to the steam flow path; a valve seat portion which is formed in the steam flow path and provided with a valve-seat-side convexly curved surface having a cross-sectional area gradually increasing toward a downstream side in a flow direction of steam in the steam flow path; and a seat portion which is formed in the valve body and has, at a site facing the valve seat portion, a seat-side convexly curved surface having an outer diameter gradually increasing toward the downstream side from an upstream side in the flow direction of the steam, wherein in the seat-side convexly curved surface, an average radius of curvature Rv in the flow direction from a starting end portion on the upstream side in the flow direction of the steam to a termination end portion on the downstream side in the flow direction of the steam has a relationship of Rv<Rs with respect to a radius of curvature Rs in the flow direction of the steam of the valve-seat-side convexly curved surface, characterized in that the seat-side convexly curved surface is provided with an upstream-side curved surface on the upstream side in the flow direction of the steam, and a downstream-side curved surface on the downstream side in the flow direction of the steam, the upstream-side curved surface and the downstream-side curved surface having radii of curvature different from each other, the downstream-side curved surface has an abutting area abutting the valve seat portion, and a radius of curvature R1 of the downstream-side curved surface that extends in the flow direction of the steam has a relationship of Rs<R1 with respect to the radius of curvature Rs in the flow direction of the steam. a casing with a steam flow path formed therein; a valve body disposed in the steam flow path and provided so as to be capable of moving relative to the steam flow path; a valve seat portion which is formed in the steam flow path and provided with a valve-seat-side convexly curved surface having a cross-sectional area gradually increasing toward a downstream side in a flow direction of steam in the steam flow path; and a seat portion which is formed in the valve body and has, at a site facing the valve seat portion, a seat-side convexly curved surface having an outer diameter gradually increasing toward the downstream side from an upstream side in the flow direction of the steam, wherein in the seat-side convexly curved surface, an average radius of curvature Rv in the flow direction from a starting end portion on the upstream side in the flow direction of the steam to a termination end portion on the downstream side in the flow direction of the steam has a relationship of Rv<Rs with respect to a radius of curvature Rs in the flow direction of the steam of the valve-seat-side convexly curved surface, characterized in that the seat-side convexly curved surface is provided with an upstream-side curved surface on the upstream side in the flow direction of the steam, and a downstream-side curved surface on the downstream side in the flow direction of the steam, the upstream-side curved surface and the downstream-side curved surface having radii of curvature different from each other, the downstream-side curved surface has an abutting area abutting the valve seat portion, and a radius of curvature R1 of the downstream-side curved surface that extends in the flow direction of the steam has a relationship of Rs<R1 with respect to the radius of curvature Rs in the flow direction of the steam.