Abstract: To achieve an LSI automated design satisfying a required function and a required performance with the reuse of existing design data, there are disposed: library element memory means for storing, in the form of elements, existing design data (circuit data) together with design procedures (conversion information) thereof; element arrangement entering means for entering an element arrangement for achieving the desired function; library element specializing means for determining the functions of general-purpose elements; element synthesizing means for synthesizing an element having one function in which the functions of the elements are combined with one another; design data memory means for storing a variety of data; conversion instruction entering means for entering a design target level; conversion method selecting means for selecting a method of converting the elements to the target level, based on information relating to the design procedure of each of the elements; and design data converting means for execu