This underpants type disposable diaper (50B) comprises an absorbent body (2) and an outer body (3) which is disposed on a non-skin-contacting surface side of the absorbent body (2) and fastens the absorbent body (2), wherein the left and right side edges of the outer body (3) of a front part (F) and the left and right side edges of the outer body (3) of a rear part (R) are joined to form a pair of side seal parts (4,4), a waist opening part (8) and a pair of leg opening parts (9,9). The side seal part (4) is difficult to recognize by visual observation when wearing the diaper (50B) and thus a side seal part visibility improvement means (level difference) (53) which is capable of improving visibility of the side seal part (4) by visual observation is provided in the vicinity of the side seal part (4).
