PURPOSE: To obtain a small low cost energy analyzer which can be operated only with a residual gas usually existing in the analyzer without any need of the introduction of an operation gas used for ionization of high-speed particles.

CONSTITUTION: The hollow center 37 of a tubular incidence electrode 34 which is installed adjacent to a first cathode 23, is coaxial with the penertrating hole 31 of the first cathode 23. The hollow center 38 of a cylindrical emission electrode 35 which is installed adjacent to a second cathode 24, is coaxial with the penetrating hole 32 of the second cathode 24. A high-speed neutral particle flow 17 traveling from a plasma device, after passing through the hollow center 37 of the incidence electrode 23, becomes incident upon an area located in the space surrounded by the two cathodes 23 and 24 and an anode 21 as a beam. Of the neutral particles which form the high-speed neutral particle beam 17, those which are ionized in the area of electric discharge, after passing through the penetrating hole 32 of the second cathode 24 and the hollow center 38 of the emission electrode 35, form an ion beam 18, which is discharged outside an electric discharge device. The ion beam 18 discharged from the emission electrode 35, after passing through the hole of a conductive cylinder 33, reaches an ionic energy analyzer installed apart from the emission electrode 35, where energy analysis is carried out.