24 so that the flying direction of the ion beam 20 is parallel to the normal of the semiconductor wafer 21 as shown in fig1 that is in accordance with an instruction from the controller 25 the stage driving mechanism 24 changes the posture position of the stage 22 so that the flying direction of the ion beam 20 is parallel to the normal of the semiconductor wafer 21 in the description of fig8 to 11 for simplification the ion beam 20 comprises one ion beam having a single amount of eccentricity of the center of gravity and the ion beam passes through the one opening in the slit in the description below the ion beam 20 comprises two ion beams having different amounts of eccentricity of the center of gravity and the ion beams pass through the different openings of the slit fig1 is a diagram schematically showing how the ion beam 20 containing an ion beam 20 1 with no eccentricity of the center of gravity and an ion beam 20 2 with a certain amount of eccentricity of the center of gravity passes through the slits 25 u and 25 d the amount of current in the ion beam 20 1 entering the opening 28 b is substantially the same as the amount of current in the ion beam 20 2 entering the opening 28 c in fig1.