In an irradiation system with an ion beam/charged particle beam having an energy filter, the energy filter is formed by deflection electrodes and a deflection magnet which can be switchingly used. The deflection magnet has a general window-frame shape and is formed with a hollow portion at its center. The deflection electrodes are installed, along with suppression electrodes, in a vacuum chamber arranged in the hollow portion of the deflection magnet. The deflection electrodes are installed with respect to the deflection magnet such that a deflection trajectory of a beam caused by a magnetic field and a deflection trajectory of a beam caused by an electric field overlap each other. Since the deflection electrodes and the deflection magnet can be switchingly used, the system can deal with a wider range of beam conditions and thus is widely usable.