To obtain a private network design supporting method speedily calculating a frame loss with minimum calculation quantity by calculating an arriving rate and a band occupation rate to determine an estimate.

The skeletal plan generation part 420 of a designing part 400 prepares a network skeletal plan and stores the information in a designing result storing part 280. At the time of obtaining the skeletal plan, by the instruction of a control part 100, a design data storing part 220 stores a peak arriving rate and an average arriving rate to transfer to a quality calculation part 520. The ratio calculation part 526 of the quality calculation part 520 calculates the number of maximum arriving frames and the the number of average arriving frames within time necessary for transferring (r)-number of frames from the peak arriving rate and the average arriving rate, obtains the average, distribution and the cubic central moment of the number of arriving numbers after multiplexing and calculates a frame arriving rate of the transmission path from obtained values.