In an ion implanting apparatus 10 including a separation slit 20 which receives an ion beam 1 having passed through a mass-separation electromagnet 17 and allows a desired type of ion to selectively pass therethrough, the separation slit 20 is operable to vary a shape of a gap through which the ion beam 1 passes. In addition, the ion implanting apparatus 10 includes a variable slit 30 which is disposed between an extraction electrode system 15 and the mass-separation electromagnet 17 so as to form a gap through which the ion beam 1 passes and is operable to vary a shape of the gap so as to shield a part of the ion beam 1 extracted from the ion source 12. The ion implanting apparatus 10 may include both or one of the separation slit 20 and the variable slit 30.