A 2-shaft gas turbine can suppress an increase in NOx emission from a combustor even when degradation of compressor efficiency occurs due to a decrease in performance with long-term use. The 2-shaft gas turbine has a gas generator and a power turbine, and includes a controller configured to set, in accordance with a degree of degradation of compressor efficiency computed from measured control parameters, a target speed of the gas generator at a level lower than a target speed in a state where degradation of the compressor efficiency has not occurred. The controller is configured to control the rotating speed of the gas generator on the basis of a difference between the target speed thus set and an actual rotating speed.