To restrain generation of measurement errors of ion-beam currents by an ion-beam current measurement device, even if it is fitted at a downstream side of a plasma generating device.

The ion beam irradiation device is provided with a plasma generating device 24 and an ion beam current measurement device 30 fitted at its downstream side. The ion-beam current measurement device 30 is equipped with a beam current detector 32, and a first suppression electrode 34, as well as, a second suppression electrode 36. It is further equipped with a first suppression power source 40 impressing a negative first suppression voltage V1on the first suppression electrode 34; a second suppression power source 42 outputting a positive second suppression voltage V2;and a switching unit 44 for connecting the second suppression electrode 36 with the second suppression power source 42, when the plasma generating device 24 is turned on, and for separating the second suppression electrode 36 from the second suppression power source 42 and connecting it with the first suppression power source 40, when the plasma generating device 24 is turned off.

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