PURPOSE: To enable the utilization of an injection molding method easy to control thickness by preventing the diameter of the peak of each of bellows turns from becoming larger than the diameter of the trough connecting bellows turns to the bellows turns adjacent to the large diameter end of a bellows by a specific ratio or more.

CONSTITUTION: Each bellows turn 10A has the relatively steeply inclined front face 19A guided to the peak 16A thereof and the flat face 20A going toward the trough 18A between the bellow turn and the next turn nearer to the large diameter end of a bellows from the peak 16A. The deformed shapes of these faces 19A, 20A can be demolded only by the extremely slight stretching quantity of a material after the injection molding of the bellows. The diameter A of the peak 10A of each bellows turn 10A is not made larger than the diameter B of the trough 18A joining the bellows turn to the next rurn 10A near to the large diameter end 11 of the bellows by 15% or more.