radius becomes relatively thereupon. Accordingly, the extending force of the bellows, which is exerted by the rising inner pressure, is larger relatively on the side of the major-diameter fastener 102, and is smaller relatively on the side of the minor-diameter fastener 103. That is, the bellows extends on the side of the major-diameter fastener 102, but contracts on the side of the minor-diameter fastener 103. Consequently, the extension of the bellows on the side of the major-diameter fastener 102 inhibits the portions of the bellows, which are disposed closer to the opposite end on the side of the major-diameter fastener 102, from moving diametrically. As a result, the conventional steering boot can inhibit the interference with one of the opposite ends of ball joint furthermore.
In addition, Japanese UnexaminedPatent Publication (KOKAI) Gazette No. 2005-147,297 discloses a steering boot in which the central section of the bellows has a root inside diameter, which is made identical with or smaller than that of the portions on the side of the major-diameter fastener 102; and the central section thereof exhibits a lower radial rigidity than that of the other portions. Namely, this conventional boot comprises a bellows which exhibits an enhanced diametric rigidity on the side of the major-diameter fastener 102, and at the same time an axial rigidity, which is held at the same level as that of the central section of the bellows, on the side of the major-diameter fastener 102. The conventional steering boot can be inhibited from undergoing the undulating deformation by such a relatively