[Problems] To improve the air permeability of a waist portion of a disposable diaper[Means for Solving Problems] In an underpants type disposable diaper, including: a ventral-side outer sheet 12F and a back-side outer sheet 12B which are not connected but separated at a crotch portion an absorber 20 having a liquid impervious sheet and a crotch outer sheet 12M laminated on an underside of the liquid impervious sheet in the absorber so as to be exposed on an external surface of a product, assuming that the number of sheets disposed on the underside of the liquid impervious sheet is B1 at the intermediate portion 23, A1 at front- and back-side outer overlap sections X and Z, and C1 at the front- and back-side outer nonoverlap sections 20F and 20B, there is established a relationship of B1
