The present invention relates to a gas turbine which includes a cooling system provided with cooling air supply paths bypassed through an outside casing and, more specifically, to a gas turbine including a cooling system and a cooling method, wherein, in order to supply cooling air to a plurality of turbine blades and other devices provided to the inside of the gas turbine, cooling air supply paths are not provided to the rotor center shaft of the gas turbine but provided around the outer casing of the gas turbine so as to achieve the increased effect in the aerodynamic efficiency of the compressor and the turbine. According to the above structure and method, it may be possible to provide the cooling air supply paths not to the rotor center shaft of the gas turbine but around the outer casing of the gas turbine, thereby finally achieving the increased effect in the aerodynamic efficiency of the compressor and the turbine.