resin material and particularly only if the small diameter pipe 110 and the large diameter pipe 120 have the sectional shapes corresponding to each other they have various sections such as circular oval and polygonal sections fig2 shows the small diameter pipe 110 the large diameter pipe 120 the upper coupler 200 and the lower coupler 300 disposed independently of each other as respective components however as shown in fig1 the small diameter pipe 110 and the upper coupler 200 may be formed unitarily 1200 with each other while the large diameter pipe 120 and the lower coupler 300 being formed unitarily 1300 with each other of course the small diameter pipe 110 and the large diameter pipe 120 may be changeably formed unitarily with the upper coupler 200 and the lower coupler 300 the extension spring 400 has the upper coupling end 410 and the lower coupling end 420 on which the fastening holes 411 and 421 are formed the extension spring 400 further has the extension piece 423 disposed on the lower end thereof in such a manner as to be exposed to the outside through the through hole 330 of the lower coupler 300 so that the extension piece 423 is pulled by a given length to allow the extension spring 400 to be easily.