To properly decrease emission of nitrogen oxide in a system and prevent excess injection of ammonia even if the load instructions for respective shafts are not even.

An affiliated denitration control apparatus 23 controls the discharge amount setting values of nitrogen oxide in a system corresponding to the load instruction values Pa, Pb, and Pc of the respective shafts and outputs the discharge amount setting values Qra0, Qrb0, and Qrc0 for lowering the discharge amounts of nitrogen oxide at the same ratio for the respective shafts. The denitration control apparatus 22a, 22b, and 22c for the respective shafts control the injection amounts of ammonium fed to the respective denitration apparatus 18a, 18b, and 18c of waste heat recovery boilers 15a, 15b, and 15c by selecting the lower values between the nitrogen oxide concentration set values Qra, Qrb, and Qrc respectively differently set for the respective shafts and the concentration set values calculated by converting the discharge amount setting values Qra0, Qrb0, and Qrc0 controlled by the affiliated denitration control apparatus 23 into the concentrations.