The present invention provides an ion group irradiation device which includes: an ion source which generates an ion; and an ion group selecting unit which selects an ion group containing a cluster ion from ions released from the ion source, in an ion group irradiation device for irradiating a sample with the ion group, wherein the ion source has a pressure gradient forming unit for changing a pressure with which a material of the cluster ion is jetted, with time, the ion group selecting unit has a chopper which performs a chopping operation of selecting the ion group by passing and blocking the cluster ions in a traveling direction by the opening and closing of the chopper, and the chopper performs two or more times of the chopping operations per one time of a pressure gradient forming operation by the pressure gradient forming unit.