The present invention generally relates to a device for adjusting girth of a waistband of pants or a skirt, in which the device can efficiently adjust the girth of a waistband of pants or a skirt in response to body changes such as a weight gain or loss. To achieve the above-mentioned object, the device of the present invention is configured as described hereinbelow. The waistband of the pants or the skirt is divided into a front waistband and a rear waistband, and the front waistband overlaps the rear waistband at an end part of the rear waistband. A guide rail is fixed to the rear waistband, and the rear waistband and the front waistband are connected to each other, and a latch engaging with a rack part is mounted to a sliding member, wherein an elastic member is fitted over a hinge pin at a middle part thereof.