To reduce electric charges on the surface of a substrate accompanying ion beam irradiation, without deteriorating the vacuum level in a beam line or a substrate processing chamber, by generating a large number of low energy electrons and utilizing them for neutralization.

This ion beam irradiation device comprises a planar negative electrode 10, which is in the same potential with a holder 6 for holding a substrate 4 and emits thermal electrons 12 by being heated with an external source, a heating source 14 for heating the electrode 10, an extraction electrode 26 for extracting thermal electrons 12 emitted from the negative electrode 10, and an extraction power source 28 for applying an extraction voltage VE of not more than 6 V on the extraction electrode 26. Additionally, the ion beam irradiation device comprises a magnetic field generator 34 for deflecting the direction of extracted thermal electrons 12 to make them advance to an ion beam 2 and to the substrate 4 side, and an electron release port 32 for selectively getting through thermal electrons 12, which move on loci of a prescribed range.