A disposable diaper, comprising: a front waistline region; a rear waistline region; a crotch region; a product longitudinal direction from the front waistline region towards the rear waistline region; a product widthwise direction that is perpendicular to the product longitudinal direction; an absorber including an absorbent core with a liquid retention property; a pair of side flaps provided at side edges of the absorber in the product widthwise direction; a fastening tape provided to each of the side flaps, projecting from the rear waistline region outwardly in the product widthwise direction, and configured to be attached to a target unit formed in the front waistline region, a pair of leg openings provided in the side flaps, and concave inwardly in the product widthwise direction; and a pair of leg stretch units expansible and contractible along the leg openings; wherein each of the leg stretch units has a part bending outwardly in the product widthwise direction as the leg stretch unit extends from the crotch region outwardly in the product longitudinal direction; a separation distance between the leg stretch units in the product widthwise direction in the crotch region is smaller than separation distances between the leg stretch units in the product widthwise direction in the front waistline region and the rear waistline region; the separation distance in the rear waistline region is longer than the separation distance in the front waistline region; the leg stretch units extending from the crotch region towards the rear waistline region are convex outwardly in the product widthwise direction; and the leg stretch units extending from the crotch region towards the front waistline region are convex inwardly in the product widthwise direction. a front waistline region; a rear waistline region; a crotch region; a product longitudinal direction from the front waistline region towards the rear waistline region; a product widthwise direction that is perpendicular to the product longitudinal direction; an absorber including an absorbent core with a liquid retention property; a pair of side flaps provided at side edges of the absorber in the product widthwise direction; a fastening tape provided to each of the side flaps, projecting from the rear waistline region outwardly in the product widthwise direction, and configured to be attached to a target unit formed in the front waistline region, a pair of leg openings provided in the side flaps, and concave inwardly in the product widthwise direction; and a pair of leg stretch units expansible and contractible along the leg openings; wherein each of the leg stretch units has a part bending outwardly in the product widthwise direction as the leg stretch unit extends from the crotch region outwardly in the product longitudinal direction; a separation distance between the leg stretch units in the product widthwise direction in the crotch region is smaller than separation distances between the leg stretch units in the product widthwise direction in the front waistline region and the rear waistline region; the separation distance in the rear waistline region is longer than the separation distance in the front waistline region; the leg stretch units extending from the crotch region towards the rear waistline region are convex outwardly in the product widthwise direction; and the leg stretch units extending from the crotch region towards the front waistline region are convex inwardly in the product widthwise direction.