Problem is to prevent appearance from deteriorating due to a conspicuous corner of a side edge of a gluteal cover portion on a side of a leg opening in an outer member separated type underpants-type disposable diaper.The above-mentioned problem is solved by an underpants-type disposable diaper separately having a front side outer member 12F and a back side outer member 12B, the front side outer member 12F and the back side outer member 12B being spaced apart in a front-back direction LD in a middle in the front-back direction LD, in which the back side outer member 12B has a gluteal cover portion C extending to a center side of the side seal portions 12A in the front-back direction, and a front-back direction dimension of a side edge of the gluteal cover portion C is 0.9 to 1.1 times a width direction dimension W3 from a side edge of the back side outer member 12B to a side edge of a center side of the side seal portions 12A in the width direction.
