PURPOSE: To reduce the number of elements to be used for a logical circuit ad to reduce the number of signal passing elements by forming signal line data from logical circuit data and determining the polarity of a signal line from polarity candidate data to control the polarity of the signal line.

CONSTITUTION: A polarity control module 301 determines and controls the polarity of a signal line by using data stored in data storage parts 302 to 304. The function block/logical circuit data storing part 302 stores logical circuit data obtained by logically converting (synthesizing) a function and function block data. The polarity candidate data storing part 303 stores the candidates of the polarity patterns of an I/O signal to be obtained at the time of executing the logical conversion (synthesis) of each function block. A signal line data storage part 304 stores the data (including the determined polarity) of the signal line. Consequently, the number of elements (inverter or the like) to be used for the logical circuit can be reduced and the number of signal passing elements (stages) in the logical circuit can be reduced.