PURPOSE: To obviate the limitation to the number of times of mill motor startings and stoppings and to the interval time from starting to the next starting and reduce much of the limitation to the number of times of load variations and to the width of the load variations of a plant as a whole by conducting all time warming and operating without stopping the mill motor.

CONSTITUTION: If a press-down button 6 to use all time warming is selected, an AND 7 is established even if a coal supply machine 12 is stopped by the instruction 5 to stop the coal supply machine 12 when the mill outlet temperature reaches below a specified temperature, and the instruction 8 to open an air purge valve is issued. Then an air purge valve 18 opens and closes again after the purge for a specified time is finished. Then a mill purge instruction 9 is issued, and the mill purge is carried out for a specified time with a cold air damper 14 fully opened, and when it is finished, an all time warming instruction 10 is issued with the mill in operation and the all time warming valve is opened. And the all time warming valve 17 controls so as to bring the outlet temperature of the mill to a specified temperature with the cold air damper 14 at a specified opening.