PURPOSE: To cool a magnetic core efficiently by installing a means to inject the refrigerant in between a winding and the magnetic core.

CONSTITUTION: Long and narrow spaces are formed between wires of a winding 2 of a flat conductor coated with an insulator. Then, pipes 3, 4, 5 are inserted into the spaces and the refrigerant is forcibly injected between the winding 2 and a magnetic core 1 through the pipes 3, 4, 5 to cool the magnetic core 1. Furthermore, spaces are made between the magnetic core 1 and the winding 2 by placing insulators 6, 7, 8, 9 to cool the magnetic core 1. Due to this structure, the spaces between the wires of the winding 2 can be made small and the refrigerant can be caused to flow on the surface of the magnetic core rapidly. Therefore, the magnetic core can be cooled efficiently with a floating inductance being kept small.