An underpants-type disposable diaper separately has a front side outer member and a back side outer member, the front side outer member and the back side outer member being spaced apart in a front-back direction in a middle in the front-back direction The back side outer member has a gluteal cover portion extending to a center side of the side seal portions in the front-back direction, and a front-back direction dimension of a side edge of the gluteal cover portion is 0.9 to 1.1 times a width direction dimension from a side edge of the back side outer member to a side edge of a center side of the side seal portions in the width direction.
