Two-shaft gas turbine

A 2-shaft gas turbine is provided that can suppress an increase in NOx emission from a combustor (2) even when degradation of a compressor efficiency occurs due to decrease in performance with long-term use. The 2-shaft gas turbine having a gas generator (4) and a power turbine (5) includes: means for measuring control parameters of the compressor (1); and a controller (400) configured to set, in accordance with the degree of the degradation (”·) of the compressor efficiency computed from the measured control parameters, a target speed (NB) of the gas generator at a level lower than a target speed (N0) in a state where degradation of the compressor efficiency has not occurred, and to control the rotating speed of the gas generator on the basis of a difference between the target speed (NB) thus set and an actual rotating speed (N1).