[en] Unneutralized ion beam in JT-60 neutral beam injector will be magnetically bent by the reflecting magnet and will impinge on the surface of the water jacket protecting the magnet and on the surface of the beam dump. When the JT-60 Tokamak operates, leakage magnetic field generated by the poloidal currents will be superposed on the above magnetic field. As a result, the trajectory of the ion beam will be displaced and the heat loading of the water jacket and the beam dump will exceed a permissible level unless careful attention is paid to the magnetic field design. As one of the countermeasures against the leakage field, we first considered the magnetic shielding. After the numerical and experimental study, however, it was found difficult to obtain the satisfactory shielding factor due to lack of space. As another countermeasure, the cancellation of magnetic field by a set of additional coils (Cancellation Coils) was proposed and studied. the result showed that the cancellation was effective to reduce the leakage field to a sufficiently low value. Therefore we decided to adopt it in the JT-60 NBI design. (author)