To obtain high-directivity, stable, and high-intensity ion beam.An ion beam irradiator 10 is constituted by a combination of a laser-driven ion/electron generator 20 and an ion transporter 30 and is configured to guide ion beam with low directivity emitted from the ion/electron generator 20 to the output end while increasing the directivity of the ion beam or focusing the ion beam at the ion transporter 30. In the ion transporter 30, an electron absorber 33 is provided around a beamline 31 at a location on the upstream side in terms of the flow of the ion beam relative to multipole magnets 32. The electron absorber 33 is formed of a material (e.g., polytetrafluoroethylene (PTFE)) that can effectively absorb high-energy electrons. The electron absorber 33 is surrounded by an X-ray shield 34 made of heavy metal such as lead.