A disposable diaper 10, comprising: a front waistline unit 20; a back waistline unit 30; a crotch unit 25; a vertically long absorbent body 15 including a liquid-retaining absorber 40; a side flap 70 provided at each side edge in a product widthwise direction of the absorbent body; a leg hole opening unit 35 that is concaved towards a center in the product widthwise direction of the absorbent body, and is formed at the each side edge in the product widthwise direction of the absorbent body; wherein the disposable diaper includes a sheet-like leg hole stretch unit 100 provided at a side end, in the product widthwise direction, of the side flap, extending in a product longitudinal direction, and being partially stretchable at least in the product longitudinal direction; a side edge, in the product widthwise direction, of the leg hole stretch unit is in an undulating shape in the product longitudinal direction; a distance between the side edge, in the product widthwise direction, of the leg hole stretch unit and a straight line passing through a center, in the product widthwise direction, of the disposable diaper and extending in parallel to the product longitudinal direction, changes from the front waistline unit to the back waistline unit; a plurality of concave units where an amount of change along the distance starts increasing instead of decreasing are provided closer to the back waistline unit than a center, in the product longitudinal direction, of the disposable diaper; and a width, in the product widthwise direction, of the leg hole stretch unit provided with the concave units is smaller than a width, in the product widthwise direction, of the leg hole stretch unit other than the concave units. a front waistline unit 20; a back waistline unit 30; a crotch unit 25; a vertically long absorbent body 15 including a liquid-retaining absorber 40; a side flap 70 provided at each side edge in a product widthwise direction of the absorbent body; a leg hole opening unit 35 that is concaved towards a center in the product widthwise direction of the absorbent body, and is formed at the each side edge in the product widthwise direction of the absorbent body; wherein the disposable diaper includes a sheet-like leg hole stretch unit 100 provided at a side end, in the product widthwise direction, of the side flap, extending in a product longitudinal direction, and being partially stretchable at least in the product longitudinal direction; a side edge, in the product widthwise direction, of the leg hole stretch unit is in an undulating shape in the product longitudinal direction; a distance between the side edge, in the product widthwise direction, of the leg hole stretch unit and a straight line passing through a center, in the product widthwise direction, of the disposable diaper and extending in parallel to the product longitudinal direction, changes from the front waistline unit to the back waistline unit; a plurality of concave units where an amount of change along the distance starts increasing instead of decreasing are provided closer to the back waistline unit than a center, in the product longitudinal direction, of the disposable diaper; and a width, in the product widthwise direction, of the leg hole stretch unit provided with the concave units is smaller than a width, in the product widthwise direction, of the leg hole stretch unit other than the concave units.