A 2-shaft gas turbine comprising: a gas generator including a compressor provided with an inlet guide vane at an intake, a combustor for burning air compressed in the compressor and fuel to generate combustion gas, and a high-pressure turbine rotated by the combustion gas from the combustor to drive the compressor; a power turbine rotated by discharge gas from the high-pressure turbine to drive load equipment; means for measuring control parameters of the compressor, wherein the means for measuring control parameters includes means for detecting an outlet temperature of the compressor, and wherein the means for measuring control parameters further includes any or all of means for detecting an inlet temperature, an inlet pressure, and an output pressure of the compressor, or wherein the means for measuring control parameters includes means for detecting an outlet temperature of the compressor, and wherein the means for measuring control parameters further includes any or all of means for detecting an angle of an inlet guide vane of the compressor, a rotating speed of the gas generator, and an outlet pressure of the compressor; and a controller configured to control rotating speed of the gas generator on the basis of a difference between a target speed and an actual rotating speed of the gas generator, characterized in that the controller is further configured to, in accordance with a degree of a compressor efficiency degradation (Δη) due to a decrease in performance with long-term use which degree is computed from the control parameters, set the target speed at a level lower than the target speed in a state where the compressor efficiency degradation has not occurred. a gas generator including a compressor provided with an inlet guide vane at an intake, a combustor for burning air compressed in the compressor and fuel to generate combustion gas, and a high-pressure turbine rotated by the combustion gas from the combustor to drive the compressor; a power turbine rotated by discharge gas from the high-pressure turbine to drive load equipment; means for measuring control parameters of the compressor, wherein the means for measuring control parameters includes means for detecting an outlet temperature of the compressor, and wherein the means for measuring control parameters further includes any or all of means for detecting an inlet temperature, an inlet pressure, and an output pressure of the compressor, or wherein the means for measuring control parameters includes means for detecting an outlet temperature of the compressor, and wherein the means for measuring control parameters further includes any or all of means for detecting an angle of an inlet guide vane of the compressor, a rotating speed of the gas generator, and an outlet pressure of the compressor; and a controller configured to control rotating speed of the gas generator on the basis of a difference between a target speed and an actual rotating speed of the gas generator, characterized in that the controller is further configured to, in accordance with a degree of a compressor efficiency degradation (Δη) due to a decrease in performance with long-term use which degree is computed from the control parameters, set the target speed at a level lower than the target speed in a state where the compressor efficiency degradation has not occurred.