To make thermal power adaptable to requirements of steam power with comparative accuracy.

A fire grate 1 comprises a plurality of lower-side blowers 7.1 to 7.5 for executing a method of controlling thermal power of an incineration plant. To determine a control signal, a blower 7.2 comprises, at the starting end of a main fuel region, an appropriate measuring device, that is, a temperature sensor 17 and a pressure sensor 19, and an air supply pipe 8.2 comprises a quantity-measuring device 18. Another pressure sensor 20 is provided in a combustion chamber 3 for measuring differences in static pressure between the lower-side blowers and the combustion chamber. Values measured by the measuring device are supplied to a central processing unit ZR, which outputs a control signal to a controller RE in consideration of a flow rate coefficient according to circumstances. The controller RE connects with various adjusters 21 to 24.