PROBLEM TO BE SOLVED: To prevent a side flap from becoming "prone to fold" inside and the side flap from folding inside a diaper body while the side flap is unfolded and put on the body of a wearer upon exchanging diaper.SOLUTION: A tape type disposable diaper has a diaper body 1 and back body side flaps 2 which are provided for left and right both sides edge parts of a back body of the diaper body 1 and are equipped with fixing tapes 21 at end parts. Each of the back body side flaps 2 is folded toward a skin non-abutment surface side of the diaper body 1. Each of the back body side flaps 2 is separably joined on the skin non-abutment surface side of the diaper body 1.