1. A structural optimization system comprising: a part for changing computer-aided design model which receives a computer-aided design model and a design variable and changes the computer-aided design model according to the design variable and outputs it; a part of calculation which receives a computer-aided design model from the part for changing computer-aided design model and calculates a natural vibration mode and a natural frequency and outputs them; a part for making natural vibration mode image which receives a natural vibration mode from the part of calculation and makes a natural vibration mode image as a visualized form of the natural vibration mode and outputs it; a part of binarization which receives a natural vibration mode image from the part for making natural vibration mode image and binarizes the natural vibration mode image by a threshold value and outputs a binary image; a part for extracting pattern information which receives a binary image from the part of binarization and extracts pattern information from the binary image and outputs it together with the binary image; a part for distinguishing natural vibration mode which receives a binary image from the part of binarization and pattern information from the part for extracting pattern information and distinguishes the natural vibration mode to be tracked from natural vibration modes after computer-aided design model change using the pattern information and outputs it; and a part for calculating optimum solution which receives a natural vibration mode from the part for distinguishing natural vibration mode and finds a design variable which makes the natural frequency of the natural vibration mode a target value and outputs it.