An IGV opening command is set based on an IGV opening A set by an
exhaust gas temperature control section, an IGV opening B set by a compressor surge protective control section, and an IGV opening C set by an
exhaust gas pressure protective control section. The value of the IGV opening A, or the value of the IGV opening B, whichever is higher, is selected, the higher selected value, or the value of the IGV opening C, whichever is lower, is selected, and the lower selected value is set as the IGV opening command (gas
turbine back pressure rise avoidance priority mode). Alternatively, the value of the IGV opening A, or the value of the IGV opening C, whichever is lower, is selected, the lower selected value, or the value of the IGV opening B, whichever is higher, is selected, and the higher selected value is set as the IGV opening command (compressor surge avoidance priority mode). Alternatively, a switcher switches between these
modes.