Potentials at a plurality of points on a diameter of a semiconductor wafer 13 are measured actually. Then, a potential distribution on the diameter is obtained by spline interpolation of potentials between the actually-measured points adjacent in the diameter direction. Thereafter, a two-dimensional interpolation function regarding a potential distribution in the semiconductor wafer 13 is obtained by spline interpolation of potentials between points adjacent in the circumferential direction around the center of the semiconductor wafer 13. Then, a potential at a observation point on the semiconductor wafer 13 is obtained by substituting the coordinate value of this observation point into the two-dimensional interpolation function. As a result, a potential distribution due to electrification of the wafer can be estimated accurately, and the retarding potential can be set to a suitable value.