An ion implantation method includes measuring a beam energy of an ion beam that is generated by a high-energy multistage linear acceleration unit operating in accordance with a tentative high-frequency parameter, adjusting a value of the high-frequency parameter based on the measured beam energy, and performing ion implantation by using the ion beam generated by the high-energy multistage linear acceleration unit operating in accordance with the adjusted high-frequency parameter. The tentative high-frequency parameter provides a value different from a value of the high-frequency parameter for achieving a maximum acceleration in design to a high-frequency resonator in a part of stages including at least a most downstream stage. The adjusting includes changing at least one of a voltage amplitude and a phase set for the high-frequency resonator in the part including the at least most downstream stage.