1. A method of automatically setting a coordinate conversion factor between a first coordinate system which is fixed with respect to an edge contour of a sample and a second coordinate system which is fixed with respect to an observing unit, the observing unit being operative to produce an image of sample edge contour, the sample being mounted on a holding stage which is movable relative to the observing unit, the sample edge contour having a circular portion and the sample size and contour being known, the method comprising the steps of: a) placing the sample on the holding stage; b) moving the stage to bring a portion of the sample which includes at least a part of the contour circular portion to a position for observation by the observing unit and obtaining an image of the part of the contour circular portion; c) deriving data representing the location of points on the circular contour portion with respect to the second coordinate system; d) deriving data representing a straight line approximating the circular contour portion and data representing a selected point on the straight line which corresponds to a point at the center of the circular contour portion; e) deriving data representing a point on the circular contour portion which corresponds to the selected point on the straight line; f) repeating steps b)-e) for successive positions for observation to derive data representing points associated with different parts of the circular contour portion; g) determining the radius and center of curvature of the contour circular portion on the basis of the data representing points on the circular contour portion derived in steps e) and f); and h) obtaining a coefficient for a conversion factor between the first and second coordinate systems on the basis of the determination made in step g).