To provide a manufacturing method by which working into magnetic core can be done without imposing load on working tools in the case where machining such as punching, cutting and grindstone working is applied.

An amorphous alloy ribbon of 1 to 1,000 μm thickness is formed by rapidly cooling a molten Fe-base alloy. The ribbon is heat treated at a temperature where structure relaxation occurs, worked into a shape of magnetic core and then heat-treated at a temperature higher than the crystallization temperature. By this procedure, a structure consisting of at least ≥50% crystalline phase of ≤50 nm average grain size and the balance amorphous phase can be provided.