A pattern forming method including modifying design data subjected to a first design rule check in design data of a pattern to be formed in a semiconductor substrate, 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 as pattern forming design data used in actual pattern formation, 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 as the pattern forming design data, 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.