Mechanical properties of the dragline and the capture th read for the spider Nephila clavata, called Jorougomo in Japan, were investigated and the effect of ultraviolet rays and acid rain on their strength was studied to consider the applicability to the environmental assessment of the habitat of the spider. It was shown that (I) the dragline, which has a fracture stress of about 1500 MPa, a fracture strain of about 0.3 and a modulus of about 10 GPa, is almost equivalent to the best man-made fibres such as aramid fibres, (2) the load-strain curve for the capture thread is J-shaped and its fracture strain amounts to more than 200%, whereas the endurance load is much smaller than that for the dragline, (3) a wonderful mechanism for the capture thread termed the "windlass system" by Vollrath and Edmonds, was reconfirmed, and (4) the irradiation by ultraviolet rays and exposure to quasi-acid rain induced degradation of the thread.