The waveform created by a circuit simulator is selected. The input data 11 inputted by an inputting means are obtained for a point on the waveform or the waveform. The selected waveform and the input data 11 are analyzed by a waveform analyzing means 12 to create circuit parameter updating information 13. On the basis of the circuit parameter updating information 13, net list data are updated and the circuit simulator 5 is operated recursively. Thus, the circuit design capable of making a desired waveform can be realized.