PURPOSE: To obtain an apparatus with high practical utility value by a construction wherein a superconductive coil body is housed in a container provided with an impregnation agent reservoir, and an impregnation agent in the liquid phase at normal temperature and solidifys at extremely lower temperatures is filled in the container.

CONSTITUTION: A valve 9 is opened to introduce water, oil or the like 10 into a container 4 under vacuum suction to be impregnated in a superconductive coil body 2. The surplus water, oil or the like is stored in a reservoir 5 through a hole 6 and the valve 9 is then closed. The coil body 2 housed in the container 4 is immersed in liquid He 11 stored in a vessel 12, and operated in the condition it is cooled down to ultralow temperatures. The impregnation agent 10 is cooled by the coolant 11 to be solidified. Spatial changes in the container 4 caused at this time due to the difference in coefficient of contraction are absorbed by the impregnation agent 10 which moves from the reservoir 5 through the hole 6, so that the coil wire material is fixed stably. By so doing, it becomes possible to impregnate the coil body with no gap and to increase efficiency of cooling due to the coolant 11. Further, the impregnation agent is easy to be handled and a superconductive coil apparatus with high practical value can be obtained.