PROBLEM TO BE SOLVED: To improve the fitting property to a gluteal cleft.SOLUTION: The underpants type disposable diaper is characterized in that the elongate shape gluteal cleft part elastic member 42 in which at least both end part in the width direction become inclined parts 42t going waist opening part side as going to both ends in the width direction is fixed to the gluteal cleft opposing part of the parts located in the gluteal cleft opposing part and both side parts of the same while being extended in the longitudinal direction in the outside sheet 20 of the back body part B and the bent fit part 40 is formed in which the gluteal cleft opposing part comes into the gluteal cleft of the wearer by the constricting action of the gluteal cleft elastic member 42 in the longitudinal direction.
