A steam turbine comprising: a turbine rotor having a plurality of rotor forks (4a-4h) rowed in an axial direction; a turbine blade having blade forks (3a-3g) rowed in the axial direction of the turbine rotor, the blade forks (3a-3g) engaged with the rotor forks (4a-4h); a plurality of pin aligned holes (6a-6c, 7a-7c) in the blade forks and rotor forks (3a-3g, 4a-4h); and a plurality of fork pins (5a-5c) inserted through the pin aligned holes (6a-6c, 7a-7c) in the axial direction of the turbine rotor; characterized in that: a platform of the turbine blade has an axial central portion located closer to a circumferential convex side than an axial steam inlet end and an axial steam outlet end; wherein the steam turbine further includes the blade fork formed in a region where a circumferential position of the platform of the turbine blade is changed between the axial steam inlet end and the axial central portion; and a clearance (17-D1, 18-D, 23-D1, 24-D) between an inner diameter (17, 18, 23, 34) of at least one of the pin aligned holes (6a-6c) in the blade fork and a diameter (D1, D) of the fork pin varies along the axial direction of the turbine rotor; wherein the at least one of the pin aligned holes (6a-6c) in the blade fork is formed so that the clearance (17-D1) between the inner diameter of the at least one pin aligned hole at the steam inlet end of the blade fork and the diameter of the fork pin is formed greater than the clearance (18-D) between the inner diameter of the at least one pin aligned hole at a portion of the blade fork that differs in axial position and the diameter of the fork pin. a turbine rotor having a plurality of rotor forks (4a-4h) rowed in an axial direction; a turbine blade having blade forks (3a-3g) rowed in the axial direction of the turbine rotor, the blade forks (3a-3g) engaged with the rotor forks (4a-4h); a plurality of pin aligned holes (6a-6c, 7a-7c) in the blade forks and rotor forks (3a-3g, 4a-4h); and a plurality of fork pins (5a-5c) inserted through the pin aligned holes (6a-6c, 7a-7c) in the axial direction of the turbine rotor; characterized in that: a platform of the turbine blade has an axial central portion located closer to a circumferential convex side than an axial steam inlet end and an axial steam outlet end; wherein the steam turbine further includes the blade fork formed in a region where a circumferential position of the platform of the turbine blade is changed between the axial steam inlet end and the axial central portion; and a clearance (17-D1, 18-D, 23-D1, 24-D) between an inner diameter (17, 18, 23, 34) of at least one of the pin aligned holes (6a-6c) in the blade fork and a diameter (D1, D) of the fork pin varies along the axial direction of the turbine rotor; wherein the at least one of the pin aligned holes (6a-6c) in the blade fork is formed so that the clearance (17-D1) between the inner diameter of the at least one pin aligned hole at the steam inlet end of the blade fork and the diameter of the fork pin is formed greater than the clearance (18-D) between the inner diameter of the at least one pin aligned hole at a portion of the blade fork that differs in axial position and the diameter of the fork pin. a platform of the turbine blade has an axial central portion located closer to a circumferential convex side than an axial steam inlet end and an axial steam outlet end; wherein the steam turbine further includes the blade fork formed in a region where a circumferential position of the platform of the turbine blade is changed between the axial steam inlet end and the axial central portion; and a clearance (17-D1, 18-D, 23-D1, 24-D) between an inner diameter (17, 18, 23, 34) of at least one of the pin aligned holes (6a-6c) in the blade fork and a diameter (D1, D) of the fork pin varies along the axial direction of the turbine rotor; wherein the at least one of the pin aligned holes (6a-6c) in the blade fork is formed so that the clearance (17-D1) between the inner diameter of the at least one pin aligned hole at the steam inlet end of the blade fork and the diameter of the fork pin is formed greater than the clearance (18-D) between the inner diameter of the at least one pin aligned hole at a portion of the blade fork that differs in axial position and the diameter of the fork pin.