PURPOSE: To permit the optimum control as well as an estimating control by a method wherein a related temperature, humidity and the amount of radiation on the ground as well as a subterranean temperature and the internal surface temperature of a tube buried in the ground are detected while a necessary thermal analysis is applied to the resulting amounts of the detections.

CONSTITUTION: Each data of the subterranean temperature, measured by a thermometer 6, the temperature of the tube body, measured by the thermometer 7, an air temperature on the ground, measured by the thermometer 8, the humidity of the air on the ground, measured by a humidity meter 9, and the amount of sunshine, measured by a sunshine meter 10 are inputted into an operating circuit 11, a heat transfer equation is operated by a differential method, and the driving of a fan 4 is controlled in accordance with the result thereof to effect the optimum control or the estimating control of the operation characteristics of the whole of the system. The thermal behavior at the circumstance of a pipeline buried under the ground is analyzed by the differential method, therefore, the operational control thereof is made very simple.