A latency adjusting part calculates a necessary delay based on the number of FFs that are required to be inserted between respective modules through high level synthesis of a behavioral description. An input FF stage number acquiring part extracts a pin having an input that receives an FF, and acquires the number of stages of input FFs of FF reception. A latency re-adjusting part obtains an optimum delay based on the above-mentioned necessary delay and input delay. A former-stage module analyzing part detects, based on the above-mentioned synthetic log or HDL, a state having a minimum total number of FFs. An FF insertion optimizing synthesis part subjects an entire circuit to high level synthesis again based on the above-mentioned optimum delay and an FF inserting position obtained based on the state having the minimum number of FFs, to thereby obtain optimized HDL.