To provide a filament for plasma shower of an ion implanter which gives a uniform temperature along the length direction and supplies uniform thermoionic emission.

An assembled body 76 is provided with a ribbon filament 86 composed of a narrow and long main body having a decided length, width, and thickness for discharging thermoelectrons. The length is constituted of a central section and first and second end sections positioned on both sides of the central section, with the width of the central section being made broader than those of the first and second end sections, and the thickness is considerably smaller than the width of the filament 86 along the full length. The filament 86 is constituted of a single spiral coil and has first and second end sections which are respectively attached to the first and second leg sections 85 of the assembled body 76 and positioned in slots. It is preferable to constitute the filament 86 and first and second leg sections 85 of tungsten.