PURPOSE: To detect easily and surely the ununiform state in the circumferential direction of the dropping speed of a material loaded to the lower part of a furnace, by measuring the temperature of the front of tuyere.

CONSTITUTION: A contactless thermometer 11 which can measure the temperature of the front of plural tuyeres attached in the circumferential direction of a funace body is provided in a viewing window 12 of a bend part of a blast branch pipe 9. Thermometers 11 are provided for all tuyeres or plural tuyeres at intervals of a certain length in the circumferential direction. The temperature of the front of tuyeres measured by these thermometers indicates a value corresponding to the heat incoming and outgoing state of a combustion zone in the front of tuyeres. Though this heat incoming and outgoing state is varied by various factors, it is determined by a ratio O/C of ore raw materials to fuel in the top of the furnace if the other conditions are fixed. Then, the ratio O/C of the material loaded to the shaft furnace is changed by ≥10% of normal operation, and dropping of this changed loaded material to the front of tuyeres is detected as a variance of temperature by thermometers for measuring the temperature of the front of tuyeres. The differences between the time of the change of the ratio O/C and the time of the detection of the temperature change in individual tuyeres are compared with one another, and this time difference is obtained as deviation of the dropping speed of the loaded material in the circumferential direction.