To effectively prevent a wound coil from sliding down like a snowslide while the coil is wound by making the number of turns of a coil wire in the reverse direction in which the coil wire is obliquely wound downward larger than that of the coil wire in the normal direction in which the coil wire is obliquely wound upward so that the wound diameter of the coil wire may gradually become larger.

Under the control of a controller 7, a coil wire 3 is obliquely wound in both the normal and reverse directions by spirally piling up the coil wire 3. At the time of winding the coil wire 3, the number of turns of the wire 3 in the reverse direction in which the wire 3 is obliquely wound downward so that the wound diameter of the wire 3 may gradually become smaller is made larger than that of the wire 3 in the normal direction in which the wire 3 is obliquely wound upward so that the wound diameter of the wire 3 may gradually become larger. For example, the number of turns in the reverse direction is set at about 53-58 when that in the normal direction is 50. When the number of turns in the reverse direction is made larger than that in the normal direction, a reservoir is formed and the foundation of the slope of the coil is stabilized. Therefore, the coil strand 3 can be wound obliquely upward around the slope having the stabilized foundation.