To prevent sinking of a thick-wall portion of a large diameter cylinder portion and to secure required sealability by uniformly fastening when fastening with a fastening member.

In the boot including the large diameter cylinder part 11 having the thick-wall part 13 and a thin-wall part 14, a deep groove 18a is formed from the outer peripheral side toward the inner peripheral side in the thick-wall part 13 along the inner peripheral surface. A connecting part 20 for bridging fastening parts 19 is connected to the deep groove 18a. Projecting parts 21 projecting from the fastening parts 19 toward the deep groove 18a side are formed at positions away from both sides of the connecting part 20.