A turbine stator blade disposed in a working fluid flow path in a casing of a gas turbine, the turbine stator blade comprising: a blade effective part disposed in the working fluid flow path; an outer circumferential sidewall including a plate-shaped part that is connected to a radially outer end portion of the blade effective part, and hooks (121, 122) each extending radially outward and circumferentially from the plate-shaped part and having a tip engaged with the casing; and an inner circumferential sidewall connected to a radially inner end portion of the blade effective part; wherein the hooks (121, 122) include a front hook extending radially outward and circumferentially from an upstream end side of the plate-shaped part, and a rear hook extending radially outward and circumferentially from a downstream end side of the plate-shaped part, characterized in that the slit is formed at the rear hook, and the seal members include: a rear hook first seal plate in a plate shape extending on both sides of the slit to block the slit, in a width direction thereof, and extending in a radial direction from a radially outer surface of a rear hook wall part of the rear hook to a radial position of a bottom portion of the slit, in a longitudinal direction thereof; and a rear hook second seal plate extending on both sides of the slit to block the slit, in the width direction thereof, and extending in an axial direction from a rear side surface of a rear hook protruding part of the rear hook toward an upstream direction to a position that is in contact with the rear hook first seal plate, in the longitudinal direction thereof. a blade effective part disposed in the working fluid flow path; an outer circumferential sidewall including a plate-shaped part that is connected to a radially outer end portion of the blade effective part, and hooks (121, 122) each extending radially outward and circumferentially from the plate-shaped part and having a tip engaged with the casing; and an inner circumferential sidewall connected to a radially inner end portion of the blade effective part; wherein the hooks (121, 122) include a front hook extending radially outward and circumferentially from an upstream end side of the plate-shaped part, and a rear hook extending radially outward and circumferentially from a downstream end side of the plate-shaped part, characterized in that the slit is formed at the rear hook, and the seal members include: a rear hook first seal plate in a plate shape extending on both sides of the slit to block the slit, in a width direction thereof, and extending in a radial direction from a radially outer surface of a rear hook wall part of the rear hook to a radial position of a bottom portion of the slit, in a longitudinal direction thereof; and a rear hook second seal plate extending on both sides of the slit to block the slit, in the width direction thereof, and extending in an axial direction from a rear side surface of a rear hook protruding part of the rear hook toward an upstream direction to a position that is in contact with the rear hook first seal plate, in the longitudinal direction thereof. a rear hook first seal plate in a plate shape extending on both sides of the slit to block the slit, in a width direction thereof, and extending in a radial direction from a radially outer surface of a rear hook wall part of the rear hook to a radial position of a bottom portion of the slit, in a longitudinal direction thereof; and a rear hook second seal plate extending on both sides of the slit to block the slit, in the width direction thereof, and extending in an axial direction from a rear side surface of a rear hook protruding part of the rear hook toward an upstream direction to a position that is in contact with the rear hook first seal plate, in the longitudinal direction thereof.