A correction target pattern having a size not more than a threshold value is extracted from first design data containing a pattern of a semiconductor integrated circuit. The first characteristic of the semiconductor integrated circuit is calculated on the basis of the first design data. Second design data is generated by correcting the correction target pattern contained in the first design data. The second characteristic of the semiconductor integrated circuit is calculated on the basis of the second design data. It is checked whether the characteristic difference between the first characteristic and the second characteristic falls within a tolerance. It is decided to use the second design data to manufacture the semiconductor integrated circuit when the characteristic difference falls within the tolerance.