Preparation supporting software for sequence programs in a safety PLC capable of rewriting sequence programs, wherein the preparation supporting software for sequence programs judges whether or not an output section corresponding to an object sequence program is of the configuration ensured to be safe, based on input device kind information, which stores first kind information indicative of safety ensured or second kind information indicative of safety not ensured in correspondence to each input device, and the object sequence program, and displays the result of the judgment distinguishably.