A turbine comprising: a rotatable turbine rotor disk; a plurality of turbine blades disposed on the turbine rotor disk; a casing configured to accommodate the turbine blades; and a ring segment mounted in the casing, the ring segment comprising: a shield wall having an outer side that faces an inner wall of the casing and an inner side that faces an outer periphery of the turbine blade, wherein the shield wall has a cavity defined in a center of the outer side thereof, the cavity being recessed inwardly from the outer side of the shield wall to a predetermined depth; a first hook and a second hook protruding from the outer side of the shield wall toward the casing for coupling therewith, the first and second hooks extending in a circumferential direction of the turbine and accommodating a cooling fluid therebetween; an air pouch recessed from the outer side of the shield wall toward the turbine blade, wherein the air pouch is recessed further than the cavity in the cavity, and wherein one side of the air pouch is formed continuous and flush with an inner surface of the second hook, the inner surface of the second hook facing towards the first hook; and at least one first cooling hole having an inlet in communication with the air pouch and an outlet in communication with an outside of the first hook in the shield wall. a rotatable turbine rotor disk; a plurality of turbine blades disposed on the turbine rotor disk; a casing configured to accommodate the turbine blades; and a ring segment mounted in the casing, the ring segment comprising: a shield wall having an outer side that faces an inner wall of the casing and an inner side that faces an outer periphery of the turbine blade, wherein the shield wall has a cavity defined in a center of the outer side thereof, the cavity being recessed inwardly from the outer side of the shield wall to a predetermined depth; a first hook and a second hook protruding from the outer side of the shield wall toward the casing for coupling therewith, the first and second hooks extending in a circumferential direction of the turbine and accommodating a cooling fluid therebetween; an air pouch recessed from the outer side of the shield wall toward the turbine blade, wherein the air pouch is recessed further than the cavity in the cavity, and wherein one side of the air pouch is formed continuous and flush with an inner surface of the second hook, the inner surface of the second hook facing towards the first hook; and at least one first cooling hole having an inlet in communication with the air pouch and an outlet in communication with an outside of the first hook in the shield wall. a shield wall having an outer side that faces an inner wall of the casing and an inner side that faces an outer periphery of the turbine blade, wherein the shield wall has a cavity defined in a center of the outer side thereof, the cavity being recessed inwardly from the outer side of the shield wall to a predetermined depth; a first hook and a second hook protruding from the outer side of the shield wall toward the casing for coupling therewith, the first and second hooks extending in a circumferential direction of the turbine and accommodating a cooling fluid therebetween; an air pouch recessed from the outer side of the shield wall toward the turbine blade, wherein the air pouch is recessed further than the cavity in the cavity, and wherein one side of the air pouch is formed continuous and flush with an inner surface of the second hook, the inner surface of the second hook facing towards the first hook; and at least one first cooling hole having an inlet in communication with the air pouch and an outlet in communication with an outside of the first hook in the shield wall.