An underpants-type disposable diaper, comprising: an outer body (12F, 12B) having a ventral side outer body and a dorsal side outer body, side edge portions of the ventral side outer body and side edge portions of the dorsal side outer body being joined at both sides in a width direction to form a waist opening; an inner body extended from a central area of the ventral side outer body in the width direction to a central area of the dorsal side outer body in the width direction so as to pass through a wearer's crotch; and the ventral-side outer body and the dorsal side outer body not being continuous at the leg sides and are separated from each other, wherein in the outer body (12F, 12B) of at least one side of the ventral side outer body and the dorsal side outer body, a crotch side edge portion of a central portion in the width direction is joined with the inner body to form an inner body joined section, in the side in the outer body (12F, 12B) of the at least one side closer to the waist than to the inner body joined section, a separation portion composed of a slit or an elongated opening is provided so as to extend in the width direction from one side to the other side of a center in the width direction, in the crotch side of the separation portion, an elongated resilient and elastic member, which is fixed in an extended state along the width direction and which is in an oblique direction when the diaper is worn, is provided, a portion disposed at the crotch side of the separation portion is deformed to separate in relation to a portion disposed at a waist side of the separation portion to the crotch side so that a length in a front-back direction of a side in the outer body (12F, 12B) closer to the waist than to the inner body joined section is extensible to a crotch side, and while, when the diaper is used, a portion of the inner body disposed closer to the waist than to the inner body joined section is moved to the crotch side, relative to the outer body (12F, 12B) of the at least one side, the length in the front-back direction of the side in the outer body (12F, 12B) closer to the waist than to the inner body joined section is extensible to the crotch side while the inner body is moved to the crotch side relative to the outer body, so that the edges (Le) of the leg openings of the outer body (12F, 12B) of the at least one side positioned in lateral sides of the inner body obliquely face up to the side edge portions. an outer body (12F, 12B) having a ventral side outer body and a dorsal side outer body, side edge portions of the ventral side outer body and side edge portions of the dorsal side outer body being joined at both sides in a width direction to form a waist opening; an inner body extended from a central area of the ventral side outer body in the width direction to a central area of the dorsal side outer body in the width direction so as to pass through a wearer's crotch; and the ventral-side outer body and the dorsal side outer body not being continuous at the leg sides and are separated from each other, wherein in the outer body (12F, 12B) of at least one side of the ventral side outer body and the dorsal side outer body, a crotch side edge portion of a central portion in the width direction is joined with the inner body to form an inner body joined section, in the side in the outer body (12F, 12B) of the at least one side closer to the waist than to the inner body joined section, a separation portion composed of a slit or an elongated opening is provided so as to extend in the width direction from one side to the other side of a center in the width direction, in the crotch side of the separation portion, an elongated resilient and elastic member, which is fixed in an extended state along the width direction and which is in an oblique direction when the diaper is worn, is provided, a portion disposed at the crotch side of the separation portion is deformed to separate in relation to a portion disposed at a waist side of the separation portion to the crotch side so that a length in a front-back direction of a side in the outer body (12F, 12B) closer to the waist than to the inner body joined section is extensible to a crotch side, and while, when the diaper is used, a portion of the inner body disposed closer to the waist than to the inner body joined section is moved to the crotch side, relative to the outer body (12F, 12B) of the at least one side, the length in the front-back direction of the side in the outer body (12F, 12B) closer to the waist than to the inner body joined section is extensible to the crotch side while the inner body is moved to the crotch side relative to the outer body, so that the edges (Le) of the leg openings of the outer body (12F, 12B) of the at least one side positioned in lateral sides of the inner body obliquely face up to the side edge portions.