PURPOSE: To obtain isotropic magnetic properties of a toroidal core by a method wherein a magnetic field is generated by a conductor which passes through the center of the toroidal core and crosses the axis of symmetry of the core with a prescribed angle and any one of the core or the conductor is rotated keeping the axis of symmetry as a center and the angle between the conductor and the axis of symmetry is kept constant during the heat treatment.

CONSTITUTION: A reclined magnetic field is applied to a toroidal core 1 when a linear conductor 3 is provided with an angle θ against an axis of symmetry 2 of the core 1 and a current is applied to the conductor 3. If the conductor 3 or the core 1 is rotated around the axis 2 keeping the angle θ constant, an isotropic field can be applied to the core 1. Heat treatment of the core 1 is performed under this condition at the temperature not higher than crystallization temperature and Curie temperature. With this constitution, no anisotropic induction magnetism is generated in the core regardless of the kinds of amorphous metal which composes the core, while the magnetic properties of the core becomes isotropic. Thus, a toroidal core with high performance without aging deterioration of the properties can be obtained.