An underpants-type disposable diaper comprising: a rectangular front outer member forming at least a lower torso portion of a front body and a rectangular back outer member forming at least a lower torso portion of a back body independently, the front outer member and the back outer member being separated from each other at a front-back direction intermediate portion in the front-back direction; an inner member including an absorber extending in the front-back direction from the front outer member to the back outer member, and being bonded to the front outer member and the back outer member respectively; and a side seal portion to which both sides of the front outer member and both sides of the back outer member are bonded to form a waist opening and a pair of left and right leg openings, the back outer member having a gluteal cover portion extending closer to a center in the front-back direction than the side seal portion, a front-back direction length of the back outer member being 1.1 to 1.5 times a front-back direction length of the side seal portion, and the gluteal cover portion having three or more elongated cover portion elastic members (16a, 16b, 16c) along a width direction at intervals in the front-back direction at least in a region located closer to both sides in the width direction than the inner member, and the gluteal cover portion being contracted toward a center in the width direction by the cover portion elastic members (16a, 16b, 16c), wherein the cover portion elastic member located closest to the waist opening is disposed at an end portion of the gluteal cover portion on the waist opening side, and the cover portion elastic member located closest to the leg opening is disposed at an end portion of the gluteal cover portion on the leg opening side, and when a central interval between the cover portion elastic member located closest to the waist opening and the cover portion elastic member located second from the waist opening is represented by d A, and a front-back direction length of the gluteal cover portion is represented by d E, d A > 0.5d E is satisfied. a rectangular front outer member forming at least a lower torso portion of a front body and a rectangular back outer member forming at least a lower torso portion of a back body independently, the front outer member and the back outer member being separated from each other at a front-back direction intermediate portion in the front-back direction; an inner member including an absorber extending in the front-back direction from the front outer member to the back outer member, and being bonded to the front outer member and the back outer member respectively; and a side seal portion to which both sides of the front outer member and both sides of the back outer member are bonded to form a waist opening and a pair of left and right leg openings, the back outer member having a gluteal cover portion extending closer to a center in the front-back direction than the side seal portion, a front-back direction length of the back outer member being 1.1 to 1.5 times a front-back direction length of the side seal portion, and the gluteal cover portion having three or more elongated cover portion elastic members (16a, 16b, 16c) along a width direction at intervals in the front-back direction at least in a region located closer to both sides in the width direction than the inner member, and the gluteal cover portion being contracted toward a center in the width direction by the cover portion elastic members (16a, 16b, 16c), wherein the cover portion elastic member located closest to the waist opening is disposed at an end portion of the gluteal cover portion on the waist opening side, and the cover portion elastic member located closest to the leg opening is disposed at an end portion of the gluteal cover portion on the leg opening side, and when a central interval between the cover portion elastic member located closest to the waist opening and the cover portion elastic member located second from the waist opening is represented by d A, and a front-back direction length of the gluteal cover portion is represented by d E, d A > 0.5d E is satisfied.