Shutter gate grid is a structure wherein first and second disc-shaped members made of metal with an insulation sheet member sandwiched therebetween are integrally joined by means of insulative screws. Gate grids with conductive wires secured to the central openings thereof are fused onto the disc-shaped members; screw insertion holes formed in the second disc-shaped member are fan-shaped and have play in the direction of rotation, making it possible to adjust the parallelism of the conductive wires of the two members by finely adjusting the rotational position when tightening the screws. Moreover, diffusion gas can be made to flow through ventilation holes provided at common locations on each member, thus making it possible to provide uniformity of diffusion gas flow in the drift region.