PURPOSE: To perform highly reliable operation dealing with various sorts of coal by a method wherein when mono-fuel combustion of pulverized coal is performed at a minimum load, lower limit loads for various sorts of coal are determined by comparing actual results with analyzed results of raw coal based on the mill load factor.

CONSTITUTION: To control a coal feeder 13 and a primary air damper 14, the coal amount 2 of feedback signal is subtracted from a combustion instruction 1 by a subtracter 4, and demand signals for the coal feeder 13 and the primary air damper 14 are produced by a controller 5. The demand signal is transmitted to the coal feeder 13 via selector 6 and an auto-manual changeover switch 11. 011 the other hand, after the demand signal produces a primary air flow rate signal at a characteristic arithmetic device 7, a primary air flow rate 3 is subtracted from the demand signal by a subtracter 8, and the demand signal is given to the primary air damper 14 via a selector 10 and an auto-manual changeover switch 12. The properties of coal in a bunker in service at present, such as volatile matter, water content, ash content and HGI, are measured and compared with the properties of raw coal which have been stored as data base, and a signal based on minimum input, value calculated by an arithmetic device 15 is given to a selector 6, so that a lower limit value required for protecting combustion is set.