A control apparatus of an extracted air booster system of an integrated gasification combined cycle power plant can additionally serve a sufficient pressure control function and a sufficient antisurge control function. For example, when an inlet guide vane is fully closed or has a small opening degree equal to or smaller than a predetermined opening degree, a second control section of the control apparatus performs pressure control based on a pressure deviation, instead of or as well as antisurge control. In the pressure control, the degree of opening of an antisurge valve is controlled so that a booster outlet pressure detection value can become equal to a set pressure value. In a case where a pressure ratio is larger than a set pressure ratio value, it is also effective that the second control section performs the antisurge control even when the inlet guide vane is fully closed.