1. A lithography simulation 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 energy ray; obtaining the latent image of the pattern by calculation of an intensity of the energy ray; locally changing, at least in a portion corresponding to a pattern to be interested, 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 a distance between the pattern to be interested and a pattern of a neighboring region, 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 interested; and calculating a distance between intersections of a portion of the latent image curve corresponding to the pattern to be interested and the reference intensity line in the changed relative position to define an interested line width of the pattern to be interested.