An energy analysis slit of an ion implanter is configured to enable switching between a standard slit opening used for implantation processing performed under a predetermined implantation condition and a high-precision slit opening having higher energy precision than the standard slit opening and used to tune an acceleration parameter for a radio frequency linear accelerator. The acceleration parameter is determined for the predetermined implantation condition so that at least a part of ions supplied to the radio frequency linear accelerator is accelerated to have target energy, and so that the beam current amount measured by a beam measurement unit is equivalent to a target beam current amount.