PURPOSE: To provide a highly precise device parameter calculating method which can select a calculation expression based on a prescribed grammar with no intervension of manual operations nor addition of a layout pattern.

CONSTITUTION: The graphic data 1 are inputted and a device type 3 is selected. The calculation expression candidates are selected out of those designated by a user (11) in a calculation expression selection process 14. Then a device shape is analyzed in a device analysis process 5. A calculation expression is decided in a calculation expression selection process 8 if the device has a simple rectangular shape. If the device has a trapezoidal shape, the number 7 of graphics is decided and the corresponding calculation expression is decided in the process 8. Then the device parameter is calculated based on the decided calculation expression. The result of this calculation is turned into the data to be transferred to the net list processing in a net list parameter production process 10. In such a constitution, the extracting precision of the device parameter can be improved.