To smoothly install a bladder, without damaging the bladder, when inserting the bladder.

This bladder insertion method comprises a process of winding and fixing a fixing part 5c of a bladder insertion sheet 5 in a cylindrical shape on and to an outer peripheral surface of a body opening part 3 having a screw part 3a and a locking step part 3b on an inner peripheral surface, a process of covering the screw part 3a and the locking step part 3b by inserting a cover part 5d of the sheet into the body opening part 3 by folding back the bladder insertion sheet 5, a process of inserting the bladder 9 into the body opening part, a process of removing the bladder insertion sheet 5 from the body opening part 3, a process of engaging a flange part 9a of the bladder 9 with the locking step part 3b, and a process of pressing and fixing the flange part 9a by screwing a cover body 12 to the body opening part.