A seal structure for a steam turbine having a stack ring provided in an annular gap between a steam pipe and a casing of the steam turbine, wherein the stack ring comprises: at least one outer ring having an outer periphery which is in close contact with the casing; and at least one inner ring disposed alternately with the outer ring in the annular gap and having an inner periphery which is in close contact with a sleeve part of the steam pipe, wherein at least one of the outer ring is designed so that, at a normal temperature, there is a small gap between the outer ring and the inner periphery of the casing, wherein at least one of the inner ring is designed so that, at a normal temperature, there is a small gap between the inner ring and the outer periphery of the sleeve part of the steam pipe, wherein at least one of the outer ring or the inner ring has a cross sectional shape formed such that one edge (42, 44) of a virtual rectangle on an outer periphery side or an inner periphery side is partially cut off, characterized in that a ratio of L 1 to L 0 is not less than 0.3 and not greater than 0.7 where L 1 is a length of a part of the cross sectional shape of the at least one of the outer ring or the inner ring that contacts the casing or the sleeve part and L 0 is a length of the one edge (42, 44) of the virtual rectangle; wherein the sleeve part and the inner ring are each formed of martensitic steel, and the inner ring has the cross sectional shape formed such that the one edge of the virtual rectangle of the inner ring on the inner periphery side is partially cut off so that the ratio of L 1 to L 0 is not less than 0.3 and not greater than 0.7 where L 1 is the length of the part of the cross sectional shape of the inner ring that contacts the sleeve part and L 0 is the length of the one edge of the virtual rectangle. at least one outer ring having an outer periphery which is in close contact with the casing; and at least one inner ring disposed alternately with the outer ring in the annular gap and having an inner periphery which is in close contact with a sleeve part of the steam pipe, wherein at least one of the outer ring is designed so that, at a normal temperature, there is a small gap between the outer ring and the inner periphery of the casing, wherein at least one of the inner ring is designed so that, at a normal temperature, there is a small gap between the inner ring and the outer periphery of the sleeve part of the steam pipe, wherein at least one of the outer ring or the inner ring has a cross sectional shape formed such that one edge (42, 44) of a virtual rectangle on an outer periphery side or an inner periphery side is partially cut off, characterized in that a ratio of L 1 to L 0 is not less than 0.3 and not greater than 0.7 where L 1 is a length of a part of the cross sectional shape of the at least one of the outer ring or the inner ring that contacts the casing or the sleeve part and L 0 is a length of the one edge (42, 44) of the virtual rectangle; wherein the sleeve part and the inner ring are each formed of martensitic steel, and the inner ring has the cross sectional shape formed such that the one edge of the virtual rectangle of the inner ring on the inner periphery side is partially cut off so that the ratio of L 1 to L 0 is not less than 0.3 and not greater than 0.7 where L 1 is the length of the part of the cross sectional shape of the inner ring that contacts the sleeve part and L 0 is the length of the one edge of the virtual rectangle.