An underpants type disposable diaper, comprising: a front body part and a back body part which are jointed to each other at both side portions thereof to thereby form joined portions on both sides, a waist opening WO, and a pair of right and left leg openings LO; and a plurality of elongated resilient and elastic members which are fixed in an area covering from the waist opening WO to the leg openings LO at the front and back parts in an extended state in a width direction, wherein perforated lines are formed on the both side portions of the front body part or back body part so as to extend from an edge of the waist opening WO to edges of the leg openings LO, the resilient and elastic members are arranged on both sides of the perforated lines such that ends thereof facing to the perforated lines are placed at the same positions as the perforated lines or near the perforated lines, and do not extend beyond the perforated lines, and at least one pair of fastening tapes having engagement sections on inner surfaces thereof straddles the perforated lines; characterized in that the elongated resilient and elastic members are fixed only at a portion that overlaps the engagement section of the fastening tape and a portion near the overlapping portion, in an extended state in a direction that crosses the perforated line, the engagement sections are detachably engaged so as to straddle both sides of the perforated lines, and the fastening tape is detachably engaged on the external surface of the diaper by the engagement section, and is completely separated from the external surface of the diaper by releasing the engagement. a front body part and a back body part which are jointed to each other at both side portions thereof to thereby form joined portions on both sides, a waist opening WO, and a pair of right and left leg openings LO; and a plurality of elongated resilient and elastic members which are fixed in an area covering from the waist opening WO to the leg openings LO at the front and back parts in an extended state in a width direction, wherein perforated lines are formed on the both side portions of the front body part or back body part so as to extend from an edge of the waist opening WO to edges of the leg openings LO, the resilient and elastic members are arranged on both sides of the perforated lines such that ends thereof facing to the perforated lines are placed at the same positions as the perforated lines or near the perforated lines, and do not extend beyond the perforated lines, and at least one pair of fastening tapes having engagement sections on inner surfaces thereof straddles the perforated lines; characterized in that the elongated resilient and elastic members are fixed only at a portion that overlaps the engagement section of the fastening tape and a portion near the overlapping portion, in an extended state in a direction that crosses the perforated line, the engagement sections are detachably engaged so as to straddle both sides of the perforated lines, and the fastening tape is detachably engaged on the external surface of the diaper by the engagement section, and is completely separated from the external surface of the diaper by releasing the engagement.