PURPOSE: To keep the incidence angle of ion beams constant at every section in a wafer without causing the wafer to be warped, by scanning the ion beams in parallel to an x-axis direction with each of two pairs of x-axis scanning means, and then scanning the ion beams in a y-axis direction with a pair of y-axis scanning means for oscillating the wafer so that the ion beams enter it at a constant angle.

CONSTITUTION: Ion beams 4 drawn out from an ion source 2 and having gone through the processes of mass spectrometry, acceleration and the like as occasion demands, are scanned by the cooperation of two pairs of x-axis scanning electrodes 6 and 10, on which mutually reversed-polarity scanning voltages are respectively impressed from an x-axis scanning power source 20 in such a manner that the ion beams may be paralleled to an x-axis direction when given out from the pair of electrodes 10. The ion beams 4 also are scanned in a y-axis direction by a pair of y-axis scanning electrodes 8 on which scanning voltages are respectively impressed from a y-axis scanning power source 18 while a control device 24 is allowed to control an oscillating device 22 and the y-axis scanning power source 18 respectively for keeping the incidence angle of the ion beams 4 to a wafer 14 at a constant value. The incidence angle of the ion beams 4 to the wafer 14 therefore becomes constant at every section in the wafer 14.