An open-type disposable diaper comprising: a front waistline region; a back waistline region; a crotch region positioned between the front waistline region and the back waistline region; a pair of leg hole openings; an absorber spanning the crotch region and extending to the front waistline region and back waistline region; a product longitudinal direction from the front waistline region towards the back waistline region; and a product widthwise direction perpendicular to the product longitudinal direction; wherein the open-type disposable diaper further comprises: a crotch stretch unit formed in an absorber arrangement region where the absorber is arranged in the crotch region and being stretchable in the product longitudinal direction; a pair of leg stretch units formed along the leg hole openings and being stretchable in at least the product longitudinal direction; and a waist stretch unit formed in the back waistline region and being stretchable in the product widthwise direction; the absorber core arrangement region, enclosed by the crotch stretch unit, the leg stretch units, and the waist stretch unit and configuring the absorber, is formed so as not to be stretched; the length of the absorber core arrangement region in the product longitudinal direction is configured to be longer than half the length of the crotch stretch unit in the product longitudinal direction, the absorber is configured to include a low rigidity region, where a basis weight is lower than the other parts of the absorber or the absorber core does not exist, in the back waistline region; the waist stretch unit is configured to overlap at least a part of the low rigidity region in a plan view of the open-type disposable diaper; and a width of the low rigidity region in the product widthwise direction is configured to widen towards the end of the absorber on the side of the back waistline region in the product longitudinal direction. a front waistline region; a back waistline region; a crotch region positioned between the front waistline region and the back waistline region; a pair of leg hole openings; an absorber spanning the crotch region and extending to the front waistline region and back waistline region; a product longitudinal direction from the front waistline region towards the back waistline region; and a product widthwise direction perpendicular to the product longitudinal direction; wherein the open-type disposable diaper further comprises: a crotch stretch unit formed in an absorber arrangement region where the absorber is arranged in the crotch region and being stretchable in the product longitudinal direction; a pair of leg stretch units formed along the leg hole openings and being stretchable in at least the product longitudinal direction; and a waist stretch unit formed in the back waistline region and being stretchable in the product widthwise direction; the absorber core arrangement region, enclosed by the crotch stretch unit, the leg stretch units, and the waist stretch unit and configuring the absorber, is formed so as not to be stretched; the length of the absorber core arrangement region in the product longitudinal direction is configured to be longer than half the length of the crotch stretch unit in the product longitudinal direction, the absorber is configured to include a low rigidity region, where a basis weight is lower than the other parts of the absorber or the absorber core does not exist, in the back waistline region; the waist stretch unit is configured to overlap at least a part of the low rigidity region in a plan view of the open-type disposable diaper; and a width of the low rigidity region in the product widthwise direction is configured to widen towards the end of the absorber on the side of the back waistline region in the product longitudinal direction. a crotch stretch unit formed in an absorber arrangement region where the absorber is arranged in the crotch region and being stretchable in the product longitudinal direction; a pair of leg stretch units formed along the leg hole openings and being stretchable in at least the product longitudinal direction; and a waist stretch unit formed in the back waistline region and being stretchable in the product widthwise direction; the absorber core arrangement region, enclosed by the crotch stretch unit, the leg stretch units, and the waist stretch unit and configuring the absorber, is formed so as not to be stretched; the length of the absorber core arrangement region in the product longitudinal direction is configured to be longer than half the length of the crotch stretch unit in the product longitudinal direction, the absorber is configured to include a low rigidity region, where a basis weight is lower than the other parts of the absorber or the absorber core does not exist, in the back waistline region; the waist stretch unit is configured to overlap at least a part of the low rigidity region in a plan view of the open-type disposable diaper; and a width of the low rigidity region in the product widthwise direction is configured to widen towards the end of the absorber on the side of the back waistline region in the product longitudinal direction.