An underpants-type disposable diaper, in which a front side outer member disposed in a front body and a back side outer member disposed in a back body are separately provided, the front side outer member and the back side outer member are spaced apart in the front-back direction at the middle in the front-back direction, an inner member, which has a liquid pervious top sheet positioned on a front surface side thereof, a liquid impervious sheet positioned on a back surface side thereof, and an absorber interposed therebetween, extends in the front-back direction from the front side outer member to the back side outer member, and is joined to the front side outer member and to the back side outer member, side gathers extending from both sides of the inner member so as to be in contact with the legs of a wearer are included, each of the side gathers includes a base end on a side portion on a back surface side of the liquid impervious sheet in the inner member and includes a gather nonwoven fabric forming an outer surface at least from the base end to a tip end, a cover nonwoven fabric, which covers a back surface of the inner member, is provided from between the front side outer member and the inner member to between the back side outer member and the inner member, the cover nonwoven fabric extends at least from a back surface side of a base portion of one side gather to a back surface side of a base portion of the other side gather, both sides of the front side outer member and both sides of the back side outer member are joined, respectively, a waist opening and a pair of right and left leg openings are included, and elastic members are incorporated in the front side outer member and the back side outer member, wherein the cover nonwoven fabric is a perforated nonwoven fabric in which a plurality of holes penetrating a front surface and a back surface of the cover nonwoven fabric are provided at intervals entirely in the front-back direction and the width direction, and characterized in that: side edges of the cover nonwoven fabric are located at positions same as side edges of a narrowest part of the absorber in the width direction or located at positions on the center side in the width direction with respect to the side edges of the narrowest part. an inner member, which has a liquid pervious top sheet positioned on a front surface side thereof, a liquid impervious sheet positioned on a back surface side thereof, and an absorber interposed therebetween, extends in the front-back direction from the front side outer member to the back side outer member, and is joined to the front side outer member and to the back side outer member, side gathers extending from both sides of the inner member so as to be in contact with the legs of a wearer are included, each of the side gathers includes a base end on a side portion on a back surface side of the liquid impervious sheet in the inner member and includes a gather nonwoven fabric forming an outer surface at least from the base end to a tip end, a cover nonwoven fabric, which covers a back surface of the inner member, is provided from between the front side outer member and the inner member to between the back side outer member and the inner member, the cover nonwoven fabric extends at least from a back surface side of a base portion of one side gather to a back surface side of a base portion of the other side gather, both sides of the front side outer member and both sides of the back side outer member are joined, respectively, a waist opening and a pair of right and left leg openings are included, and elastic members are incorporated in the front side outer member and the back side outer member, wherein the cover nonwoven fabric is a perforated nonwoven fabric in which a plurality of holes penetrating a front surface and a back surface of the cover nonwoven fabric are provided at intervals entirely in the front-back direction and the width direction, and characterized in that: side edges of the cover nonwoven fabric are located at positions same as side edges of a narrowest part of the absorber in the width direction or located at positions on the center side in the width direction with respect to the side edges of the narrowest part.