PURPOSE: To grasp formation conditions of the stuck material in the kiln during the operation of the rotary kiln to take improvement measures of operation corresponding to them by calculating by the amount of heat from the iron skin and a the amount of heat flowed through between gas in the kiln and the iron skin.

CONSTITUTION: Outside surface temperature T2 of iron skin 1 is measured by surface thermometer 3, and the signal from this thermometer 3 is input to operator 5. Similarly, the signal from thermometer 6 for measuring of air temperature T3 is input to operator 5. Temperature T1 of the gaseous atmosphere in the kiln is measured by thermocouple 8, and the signal is input to operator 5. Operator 5 takes in signals indicating input termometric results T1, T2 and T3 as data and calculates QA, unknown number (d), and QB by equations 1 and 2 and calculates thickness (d) of stuck material 12 in the kiln by equation 3 and displays it on display equipment 11. In expressions, (d), QA, QB, r1, and K1WK5 are the thickness to be obtained of the stuck material, the amount of heat from the iron skin, the amount of heat flowed through between gas in the kiln and the iron skin, the distance (constant) from the center of the kiln to the refractory wall inside face, and constants respectively.