To provide an ion implanter which guides an ion beam having a high density of ions.

An ion source part 40 of this device comprises a chamber 42 composed of a first wall 42A and a second wall 42B, which faces to each other, and tubular side walls 42C which connect with the first wall 42A and the second wall 42B. The first wall 42A is configured as a thermoelectron emitting electrode 42A which emits thermoelectrons across its entire wall. The second wall 42B is formed so that an ion-beam guiding exit penetrates the second wall 42B. The thermoelectron emitting electrode 42A is equipped with a plurality of gas guiding inlets located with an equal spacing to each other at the vicinity of the side walls. With this configuration, even if the thermoelectron emitting electrode 42A is kept at a high temperature in order to make the electrode 42A emit a large amount of thermoelectrons and generate arc discharge having a high ion density, the side walls 42C are cooled with an introduced gas.