First, raw materials such as binder resin (e.g., 100 parts in weight of polyester) and coloring agent (e.g., 5 parts in weight of copper phthalocyanine) are blended with the charge controlling agent and other additives such as the fixability improving agent in predetermined amount and are mixed therewith in a mixing device, for example, such as a super mixer to obtain a homogeneous mixture (mixing