An ion beam irradiation method comprises calculating a scan voltage correction function with the maximum beam scan width depending on the measurement result of a beam current measurement device, calculating each of more than one scan voltage correction functions corresponding to each of scheduled beam scan widths depending on the calculated scan voltage correction functions while satisfying dose uniformity in the horizontal direction, measuring a mechanical Y-scan position during the ion implantation, changing the scan voltage correction function as a function of the measured mechanical Y-scan position so that the beam scan area becomes a D-shaped multistage beam scan area corresponding to an outer periphery of a half of the wafer to thereby reduce the beam scan width, and changing a mechanical Y-scan speed depending on the change of the measurement result of a side cup current measurement device to thereby keep the dose uniformity in the vertical direction.