PURPOSE: To enhance the productivity of a doughnut type coil and to improve the quality of the coil by winding an insulated wire in the prescribed number of turns on the outer periphery of linear magnetic core of the prescribed length, and bonding annularly both ends of the magnetic cores, thereby facilitating the mechanizing steps.

CONSTITUTION: An insulated wire 2 is wound in the prescribed number of turns on a linear magnetic coil 1 having flexibility, and the wire 2 is bonded by welding at the bonded part 3 of both ends of the core 1 thus wound with the coil. The flexible linear core 1 is used, thereby enabling the use of an automatic wire winding machine. Thus, the formation of a doughnut type closed magnetic path is facilitated with a simple work, thereby enhancing the productivity of the coil and the quality of the coil is improved.