A mechanical shaft joint boot integrally made up of a large-diameter ring portion and a small-diameter ring portion with a bellows portion interposed therebetween, and a straight portion connecting the bellows portion with the large-diameter ring portion. The distance 11 between the large-diameter ring portion and the small-diameter ring portion is smaller than the distance l2 between a large-diameter ring mounting portion and a small-diameter ring mounting portion of the joint, and the outside diameter of a first crest of the bellows portion is not larger than the inside diameter of the straight portion. Accordingly, when a shaft of the joint inclines to cause inclination on one side of the boot, a surface pressure being imposed on the first crest is not increased correspondingly, thereby suppressing its wear and enhancing service time of the boot.