PURPOSE: To keep a NOx discharge value below a basic value by dividing and arranging burners of a heater of a gas turbine into several groups, and regulating every finally cut-in group at partial loading or regulating every finally cut-in group entire loading.

CONSTITUTION: A silo heater A has a number of interior-mixing burners B, and comprises a portion for introducing compressed air A1, a channel A2 to the burners B, a portion A3 for supplying a fuel to the burners B, and a combustion 22 disposed behind the burners, to load a high-temperature A4 to a turbine. These burners B are divided into a plurality of groups. In a whole loading region, a finally cut-in group principally receives fuel supply corresponding to an ambient temperature and operates with a thin fuel. Even at partial loading, the finally cut-in group is regulated every time corresponding to ambient conditions, and this regulation is carried out via a complementary loading. Thus, a value of Nox discharge is kept below a basic value even when the ambient conditions fluctuate.