A gas turbine comprising: a casing; a compressor in the casing and configured to draw air thereinto and compress the air to a high pressure; a combustor configured to mix fuel with the air compressed by the compressor and combust a mixture of the fuel and the air; a turbine configured to rotate a plurality of turbine blades using high-temperature and high-pressure combustion gas discharged from the combustor to generate electricity, wherein the turbine comprises the plurality of turbine blades, and a plurality of turbine vanes fixed to the casing and arranged alternately with the turbine blades; a plurality of external cooling passages configured to extract air from different positions of the compressor to an outside of the casing, and supply cooling air to the turbine without having the cooling air pass through a central shaft of the turbine, wherein the plurality of external cooling passages supply air to at least one turbine vane (445, 446, 447) of the plurality of turbine vanes other than a first stage turbine vane; and an outlet cooling passage configured to extract air from an outlet of the compressor and supply cooling air to the turbine without having the cooling air pass through the central shaft of the turbine, wherein the outlet cooling passage comprises: an outlet-outside cooling passage configured to extract air from the outlet of the compressor to the outside of the casing and supply the air to the first stage turbine vane of the plurality of turbine vanes, and an outlet-inside cooling passage configured to extract air from the outlet of the compressor into the inside of the casing and supply the air to a first stage turbine blade of the plurality of turbine blades; the gas turbine further comprising at least one connection cooling passage configured to connect corresponding cooling passages of a pair structure having one turbine blade and one turbine vane, which are disposed in at least one identical stage among the turbine blades and the turbine vanes. a casing; a compressor in the casing and configured to draw air thereinto and compress the air to a high pressure; a combustor configured to mix fuel with the air compressed by the compressor and combust a mixture of the fuel and the air; a turbine configured to rotate a plurality of turbine blades using high-temperature and high-pressure combustion gas discharged from the combustor to generate electricity, wherein the turbine comprises the plurality of turbine blades, and a plurality of turbine vanes fixed to the casing and arranged alternately with the turbine blades; a plurality of external cooling passages configured to extract air from different positions of the compressor to an outside of the casing, and supply cooling air to the turbine without having the cooling air pass through a central shaft of the turbine, wherein the plurality of external cooling passages supply air to at least one turbine vane (445, 446, 447) of the plurality of turbine vanes other than a first stage turbine vane; and an outlet cooling passage configured to extract air from an outlet of the compressor and supply cooling air to the turbine without having the cooling air pass through the central shaft of the turbine, wherein the outlet cooling passage comprises: an outlet-outside cooling passage configured to extract air from the outlet of the compressor to the outside of the casing and supply the air to the first stage turbine vane of the plurality of turbine vanes, and an outlet-inside cooling passage configured to extract air from the outlet of the compressor into the inside of the casing and supply the air to a first stage turbine blade of the plurality of turbine blades; the gas turbine further comprising at least one connection cooling passage configured to connect corresponding cooling passages of a pair structure having one turbine blade and one turbine vane, which are disposed in at least one identical stage among the turbine blades and the turbine vanes. an outlet-outside cooling passage configured to extract air from the outlet of the compressor to the outside of the casing and supply the air to the first stage turbine vane of the plurality of turbine vanes, and an outlet-inside cooling passage configured to extract air from the outlet of the compressor into the inside of the casing and supply the air to a first stage turbine blade of the plurality of turbine blades; the gas turbine further comprising at least one connection cooling passage configured to connect corresponding cooling passages of a pair structure having one turbine blade and one turbine vane, which are disposed in at least one identical stage among the turbine blades and the turbine vanes.