Abstract: A beam collimator includes a plurality of lens units that are arranged along a reference trajectory so that a beam collimated to the reference trajectory comes out from an exit of the beam collimator. Each of the plurality of lens units forms a bow-shaped curved gap and is formed such that an angle of a beam traveling direction with respect to the reference trajectory is changed by an electric field generated in the bow-shaped curved gap. A vacant space is provided between one lens unit of the plurality of lens units and a lens unit that is adjacent to the lens unit. The vacant space is directed in a transverse direction of the collimated beam in a cross section that is perpendicular to the reference trajectory. An inner field containing the reference trajectory is connected to an outer field of the plurality of lens units through the vacant space.