A method for manufacturing a magnetic core module in a magnetic head, the magnetic core module and the magnetic head. The method for manufacturing the magnetic core module includes: a process for placing a magnetic core group in a holder mold cavity as an insert; and a process for injection-molding in the holder mold cavity. A method for manufacturing the magnetic core module allows the magnetic core group and the holder to be integrally injection-molded with a method of injection molding which uses the magnetic core group as an insert. The method simplifies the process of manufacturing a magnetic head to improve production efficiency, and saves labor and production costs. Further, the method prevents failures such as positional displacement and scattering of magnetic cores, which tends to occur when assembling thin and small magnetic cores, and ensures an ideal yield for a product.