To automatically delete a 'Don't touch' attribute and to prevent the delay time of a circuit generated after conversion from causing a problem by converting a cell present at a part other than a part made into a block with one or more cell in a target library having the same function.

A processing circuit 3 calculates the signal delay times between the input pins of all retrieved cells with the 'Don't touch' attribute and the output port of the circuit and stores them in a delay time data file 290. A processor 3 deletes the 'Don't touch' attributes of the cells. The processor 3 obtains the arithmetic results of the delay times and uses them as restriction conditions, optimizes a circuit part in a block by using the target library 270, and stores the result in a work file 295. Then the processor 3 converts each cell present at a part other than the part made into the block into a cell or a combination of cells in the target library 270 having the same function and stores the conversion result in the work file 295.