A disposable diaper (10, 10A, 10B) comprising: a front waistline region; a rear waistline region; a crotch region positioned between the front waistline region and the rear waistline region; a pair of leg hole opening units; an absorber running across the crotch region and extending in the front waistline region and the rear waistline region; a product longitudinal direction being from the front waistline region towards the rear waistline region; a product widthwise direction that is perpendicular to the product longitudinal direction; a waistline retaining unit (20, 30, 90) extending along the product widthwise direction in the front waistline region and the rear waistline region, and holding the disposable diaper (10, 10A, 10B) onto a body of a wearer; a crotch unit that is formed in the crotch region and can be expanded and contracted in the product longitudinal direction or the product widthwise direction; a first inflection unit that is formed in the absorber and that becomes a curved base point of the disposable diaper (10, 10A, 10B) by extending along the product widthwise direction; and a second inflection unit that is formed in the absorber, that becomes the curved base point of the disposable diaper (10, 10A, 10B) by extending along the product widthwise direction, and that is positioned towards the rear waistline region from the first inflection unit; wherein the absorber is configured to be able to bend along the first inflection unit and the second inflection unit; when the disposable diaper (10, 10A, 10B) is worn, as compared to the other portions of the absorber, the crotch unit is configured to be able to maintain a flat shape; the crotch unit is provided independently from the waistline retaining unit (20, 30, 90) without intersecting the waistline retaining unit (20, 30, 90); a size along the product longitudinal direction of the crotch unit is 30 mm or more and 150 mm or less, in a natural state of the disposable diaper (10, 10A, 10B); and a ratio of expansion and contraction of the crotch unit is 1.2 times or more and 1.8 times or less. a front waistline region; a rear waistline region; a crotch region positioned between the front waistline region and the rear waistline region; a pair of leg hole opening units; an absorber running across the crotch region and extending in the front waistline region and the rear waistline region; a product longitudinal direction being from the front waistline region towards the rear waistline region; a product widthwise direction that is perpendicular to the product longitudinal direction; a waistline retaining unit (20, 30, 90) extending along the product widthwise direction in the front waistline region and the rear waistline region, and holding the disposable diaper (10, 10A, 10B) onto a body of a wearer; a crotch unit that is formed in the crotch region and can be expanded and contracted in the product longitudinal direction or the product widthwise direction; a first inflection unit that is formed in the absorber and that becomes a curved base point of the disposable diaper (10, 10A, 10B) by extending along the product widthwise direction; and a second inflection unit that is formed in the absorber, that becomes the curved base point of the disposable diaper (10, 10A, 10B) by extending along the product widthwise direction, and that is positioned towards the rear waistline region from the first inflection unit;