To provide an expansion body for adjusting the size of a wearing article which is reduced in costs and widely usable not only for bottoms but also for the wearing article.

The expansion body for adjusting the size of a wearing article such as a size-adjusting device 100, includes a balloon 10 which is insertable into a waist part 3, and an inflater 20 which expands the balloon 10. The balloon 10 is put into the waist part 3, and the inflater 20 is connected to the balloon 10 via an injection valve 12a. A handle 24 for the inflater 20 is vertically operated to expand the balloon 10 until the waist part 3 is sufficiently depressed from the inner peripheral surface, and thereafter, the balloon 10 is kept expanded for a prescribed period of time until the diameter or the peripheral border length of the waist part 3 is elongated.