A method for developing an external plate and a process for producing an external plate in which production efficiency of the external plate can be enhanced by reducing the processing amount. In the method for developing the external plate, a first line being copied and a second line being copied are copied onto a flat plate as a first copying line and a second copying line. The actual length coordinates of the first line being copied and the geodetic curvature as a function thereof are matched to the actual length coordinates of the first copying line and the curvature as a function thereof. Relative position of the first copying line and arrangement of the second copying line in a copy region are determined such that the length of the second copying line in the copy region between adjacent first copying lines becomes not shorter than the length of a second copying line corresponding to the second copying line in the region being copied between adjacent first lines being copied corresponding to the copy region and the area of the copy region is minimized.
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