For a semiconductor device including a gate electrode in an area of part of a surface of a semiconductor substrate, a gate length is determined and to be set as an upper-limit gate length. For a semiconductor device of which a gate length is almost equal to the upper-limit gate length, an impurity implantation condition is determined to calculate a representative impurity concentration distribution. A limit gate length is obtained according to the representative impurity concentration distribution. For a semiconductor device of which a gate length is equal to or greater than the limit gate length and equal to or less than the upper-limit gate length, an impurity concentration distribution of the semiconductor device is calculated according to the representative impurity concentration distribution. Characteristics of the semiconductor device are obtained according to the impurity concentration distribution. This method reduces the period of time to calculate the characteristics of the semiconductor device.