A disposable diaper, comprising: an inner part having a longitudinal direction and a width direction orthogonal to the longitudinal direction, said inner part including a liquid permeable top sheet, a liquid impermeable back sheet, and an absorber provided between the top sheet and the back sheet; a side flap at one of end portions of the inner part in the longitudinal direction and connected to at least one of end portions of the inner part in the width direction; and a tape fastener attached to a side end portion of the side flap and for engaging with the other end portion of the inner part in the longitudinal direction; wherein the side flap includes a leg periphery side region adapted to be positioned closer to a leg of a wearer than the tape attaching part when the disposable diaper is fitted onto the wearer, characterised in that: the tape fastener includes a tape attaching part attached to the side flap and exposed at an outer side face of the side flap, the leg periphery side region is configured to be folded upwardly to cover at least partially an end portion of the tape attaching part adjacent the leg of the wearer in response to a stress generated when the tape fastener is engaged, while being pulled, with the other end portion of the inner part in the longitudinal direction, whereby said leg periphery side region folded upwardly prevents the end portion of the tape attaching part from directly contacting the wearer's skin, and wherein a length Lb of the leg periphery side region in the longitudinal direction is longer than a length La of the tape attaching part in the width direction. an inner part having a longitudinal direction and a width direction orthogonal to the longitudinal direction, said inner part including a liquid permeable top sheet, a liquid impermeable back sheet, and an absorber provided between the top sheet and the back sheet; a side flap at one of end portions of the inner part in the longitudinal direction and connected to at least one of end portions of the inner part in the width direction; and a tape fastener attached to a side end portion of the side flap and for engaging with the other end portion of the inner part in the longitudinal direction; the side flap includes a leg periphery side region adapted to be positioned closer to a leg of a wearer than the tape attaching part when the disposable diaper is fitted onto the wearer, characterised in that: the tape fastener includes a tape attaching part attached to the side flap and exposed at an outer side face of the side flap, the leg periphery side region is configured to be folded upwardly to cover at least partially an end portion of the tape attaching part adjacent the leg of the wearer in response to a stress generated when the tape fastener is engaged, while being pulled, with the other end portion of the inner part in the longitudinal direction, whereby said leg periphery side region folded upwardly prevents the end portion of the tape attaching part from directly contacting the wearer's skin, and wherein a length Lb of the leg periphery side region in the longitudinal direction is longer than a length La of the tape attaching part in the width direction. the tape fastener includes a tape attaching part attached to the side flap and exposed at an outer side face of the side flap, the leg periphery side region is configured to be folded upwardly to cover at least partially an end portion of the tape attaching part adjacent the leg of the wearer in response to a stress generated when the tape fastener is engaged, while being pulled, with the other end portion of the inner part in the longitudinal direction, whereby said leg periphery side region folded upwardly prevents the end portion of the tape attaching part from directly contacting the wearer's skin, and wherein a length Lb of the leg periphery side region in the longitudinal direction is longer than a length La of the tape attaching part in the width direction.