To provide a waist size adjusting device for adjusting the waist size of clothes such as trousers and a skirt, and especially, facilitating tucking to make a waist size smaller.

The waist size adjusting device includes a base body forming a folded part by folding both ends of a base part toward one side to have a storage space, and in which the opposite ends of the folded part are arranged with a gap, and a pinching piece formed by folding the base part in the storage space to have a pinching space.