of the first end cap electrode 182 and the second end cap electrode 184 are aligned to the axis connecting the center of the first opening 139 with the center of the second opening 187 which may be coincident with the axis of the multipole ion guide 86 a first high transmittance conductive mesh 183 and a second high transmittance conductive mesh 185 may be provided adjacent to the openings in the first end cap electrode 182 and the second end cap electrode 184 respectively the first and second high transmittance conductive meshes 183 185 encompass at least the area of the openings of the first end cap electrode 182 and the second end cap electrode 184 respectively preferably the same electric potential is applied to the first high transmittance conductive mesh 183 as to the first end cap electrode 182 and the same electric potential is applied to the second high transmittance conductive mesh 85 as to the second end cap electrode 184 the first and second high transmittance conductive meshes 183 185 flatten the electric field at the ends of the multipole ion guide 186 the ratio of the area between the wires of the first and second high transmittance conductive meshes 183 185 and the area occupied by the wires of the first and second high transmittance conductive meshes 183 85 is kept as high.