A conductor having a projecting portion is formed which forms a terminal portion. An uncured prepreg including a reinforcing material is closely attached to the conductor and the prepreg is cured to form an insulating film including the reinforcing material. When the prepreg is closely attached, the prepreg is stretched by the projecting portion, so that a region of the prepreg, which is closely attached to the conductor, can be thinner than the other region of the prepreg. Then, by reducing the thickness of the entire insulating film, an opening can be formed in the portion having a smaller thickness. The step of reducing the thickness can be performed by etching. Further, it is preferable not to remove the reinforcing material in this step. The strength of a terminal and an electronic device can be increased by leaving the reinforcing material at the opening.