PURPOSE: To permit energy saving operation by a method wherein the physical property of a pipeline buried into earth, the scale of equipment utilizing geotherm and the like are operated previously and the control of the device is effected by an underground thermometer.

CONSTITUTION: The device is constituted of the pipeline 1, buried into earth to utilize geotherm, a heat medium transfer device 4 for introducing the medium into the pipeline, detectors of heat medium temperature at the entrance 2 of the pipeline on the ground and the amount of sunshine, the underground temperature detector 12, arranged at a place easy to maintain, and a data processing and operating device 8. Underground temperature distribution and pipe wall temperature are operated by the calculation of heat transfer and a heat accumulating amount as well as operating conditions are determined by the same calculation while the operation of the heat medium transfer device 4 is controlled in accordance with the operating condition. Thus, the inlet temperature 2 and the outlet temperature 3 of the heat medium as well as the amount of retrieving heat are estimated and the control of heat radiating or accumulating operation is effected effectively in accordance with the estimated amount whereby the control of energy saving operation may be permitted.