An underpants-type disposable diaper comprising an outer body constituting individually or integrally a front body F and a back body, and an absorber provided on the inner side of the outer body ranging from the front body to the back body, the outer body of the front body and the outer body of the back body being joined together at both side edges to form side seal portions, thereby forming a waist opening and a pair of right and left leg openings, wherein the outer body of the back body has a waist part defined as a front-back direction range having the side seal portions; an intermediate portion L defined as the front-back direction range forming the leg openings and wherein the outer body has an elastic sheet layer and elongated elastic members arranged along the width direction between a first sheet layer and a second sheet layer to impart elasticity in the width direction, the intermediate portion has cover parts positioned closer to lateral sides than an inner body wherein the inner body is fixed and integrated to the inner surface of the outer body and the inner body is formed by interposing the absorber between a liquid pervious face sheet and a liquid impervious back sheet, the cover parts are formed at least by stacking the elastic sheet layer between a first sheet layer and a second sheet layer, when the elastic sheet layer is extended in a width direction, the first sheet layer and the second sheet layer are joined together directly or indirectly at a large number of dot-like joint portions arranged at intervals left in the width-direction and front-back direction, and at least the front-back direction intermediate portion of the cover parts is configured such that the arrangement density of the dot-like joint portions becomes higher stepwise or continuously from the width-direction outer edges to the width-direction center of the cover parts, the cover parts curve outward in the natural-length state. the outer body of the back body has a waist part defined as a front-back direction range having the side seal portions; an intermediate portion L defined as the front-back direction range forming the leg openings and wherein the outer body has an elastic sheet layer and elongated elastic members arranged along the width direction between a first sheet layer and a second sheet layer to impart elasticity in the width direction, the intermediate portion has cover parts positioned closer to lateral sides than an inner body wherein the inner body is fixed and integrated to the inner surface of the outer body and the inner body is formed by interposing the absorber between a liquid pervious face sheet and a liquid impervious back sheet, the cover parts are formed at least by stacking the elastic sheet layer between a first sheet layer and a second sheet layer, when the elastic sheet layer is extended in a width direction, the first sheet layer and the second sheet layer are joined together directly or indirectly at a large number of dot-like joint portions arranged at intervals left in the width-direction and front-back direction, and at least the front-back direction intermediate portion of the cover parts is configured such that the arrangement density of the dot-like joint portions becomes higher stepwise or continuously from the width-direction outer edges to the width-direction center of the cover parts, the cover parts curve outward in the natural-length state.