When an engineer designs bridges, tunnels and so on, the knowledge of the structural strength is indispensable a must. But a lot of engineering students at university level do not understand the structural strength, because they did not study physics at high school. They do not understand the trigonometric functions, so they do cannot solve the truss structures which are composed of triangles. Therefore, so the teaching staffs who are in charge of the structural strength at high schools should revise the subjects, such as of high school, for example physics especially statics, the trigonometric functions and the like so on.