A method of preparing a kneaded mixture for a magnetic coating material comprises a kneading start process and a continuous kneading process. In the kneading start process, a low-viscosity composition, which is composed of magnetic layer forming materials containing ferromagnetic grains and a binder, and a solvent, is introduced into a kneading vessel of a two-shaft continuous kneading and mixing machine, and kneading is started. In the continuous kneading process, after the start of the kneading, a high-viscosity composition, in which a solvent amount is smaller by at least 7 wt % than the solvent amount in the low-viscosity composition used in the kneading start process, is continously introduced into the kneading vessel, and the kneading is continued while high shearing force is being given to the high-viscosity composition.