A method of fabricating a reactor composed of a coil, a core, and a container, capable of suppressing the core to break when a current flows in the coil to generate magnetic flux. In the method, the coil is formed by spirally winding a conductive wire. The coil is immersed in an insulating film in liquid with electrical insulation. The coil is placed in a furnace. Annealing for the coil and thermosetting for the insulating film are performed at a temperature within 250 to 320° C. for a period of time within 30 minutes to one hour before forming the core in the container. The coil is then disposed in the container. Inside and outside areas of the coil in the container is filled with a resin mixture composed of magnetic powder and resin. The resin mixture in the container is hardened to form the core.