1. A mask pattern preparation method comprising: taking in design data of a pattern to be formed on a substrate and mask data to prepare a mask pattern used in forming a latent image of the pattern on the substrate by transmission of an energy ray, the mask pattern having a width thereof defined; calculating a distance between a pattern to be corrected and a pattern of a neighboring region of the pattern to be corrected on the basis of the design data; obtaining a latent image of the pattern to be formed on the substrate by calculation of an intensity of the energy ray; locally changing, at least in a portion corresponding to the pattern to be corrected, a relative position in a direction of the intensity of the energy ray between a latent image curve and a reference intensity line in accordance with the calculated distance, the latent image curve being an intensity distribution curve of the energy ray constituting the latent image, the reference intensity line being defined to specify a position of an edge of the pattern to be corrected; and calculating a distance between intersections of a portion of the latent image curve corresponding to the pattern to be corrected and the reference intensity line in the changed relative position to define the calculated distance as an interested line width of the pattern to be corrected; forming the pattern on the substrate to measure an actual line width of the pattern to be corrected; calculating a difference between the calculated interested line width and the measured actual line width; comparing a predetermined threshold value with the difference and newly defining the line width of the mask pattern when the difference is above the threshold value; and repeating obtaining of the latent image of the pattern to newly defining of the line width of the mask pattern until the difference becomes not more than the threshold value.