1. A pattern forming method comprising: modifying design data subjected to a first design rule check in design data of a pattern to be formed in a semiconductor substrate on the basis of a modification guideline regulated by a criterion which satisfies at least one of a request on design of the semiconductor device and a request on processes of manufacturing the semiconductor device; performing the first design rule check to the modified design data again; outputting the modified design data which does not violate the first design rule in the modified design data subjected to the first design rule again as pattern forming design data used in actual pattern formation, extracting the modified design data which violates the first design rule, and performing a second design rule check having an allowable range wider than that of the first design rule to the modified design data which violates the first design rule; and outputting the modified design data which does not violate the second design rule in the modified design data subjected to the second design rule check as the pattern forming design data, extracting the modified design data which violates the second design rule, and redesigning the pattern to satisfy the second design rule or adjusting the modification guideline such that the modified design data which violates the second design rule satisfies the second design rule.