1. A method for creating a mask pattern used in multiple exposure for carrying out overlay printing of a micro-line pattern and a mask pattern a minimum line width of which is wider than a line width of the micro-line pattern to form a target pattern having a minimum line width corresponding to the line width of said micro-line pattern, said method comprising: a step of preparing data of the target pattern desired to form after the exposure; a step of carrying out a logical operation of predetermined micro-line pattern data and said target pattern data; a step of dividing a surface of the mask pattern into plural types of areas, based on the result of the logical operation; a step of setting a single light transmittance or a plurality of light transmittances required or allowed for the types of areas and grouping areas for which one light transmittance can be selected, in each light transmittance; and a step of synthesizing a synthetic pattern from grouped patterns formed in the respective light transmittances, wherein data of the synthetic pattern is used as data of the mask pattern.