The theoretical solution of the problem of neutron depolarization in magnetic materials with the presence of magnetic field is presented. The particular case, when an applied magnetic field is perpendicular to the neutron velocity, is considered. The solution shows clearly the essential role of intrinsic depolarization anisotropy with respect to neutron velocity as well as an anisotropy produced by magnetic field in a magnetic medium. The theoretical results are confirmed in experiments for the first time.