The HV bushing has a function of a bulk head between insulation gas (SF6) of 0.6 MPa and vacuum, which is needed to insulate −1 MV in vacuum in ITER heating neutral beam (NB) injection system. Mechanical analyses were carried out to verify the mechanical integrity of the HV bushing composed of five-stage double-layered insulator columns with large brazed ceramic ring and fiber reinforced plastic (FRP) ring of which non-metallic material is required to satisfy a safety factor of ≥3.5. As for the FRP ring required sustaining the pressure load, seismic load and dead weight, it was confirmed that isotropic fiber cloth FRP rings having sufficient strength against shear stress should be used. The shape and fixation area of the Kovar sleeve were modified to lower the stress at the brazing area having vacuum leak tightness and pressure tightness against the air filled at 0.6 MPa. As a result, a design of the insulator for the HV bushing was established satisfying the requirement.