This underwear-type absorbent article (1) is equipped with a stomach-side sheet member (2A), a back-side sheet member (2B), and an absorbent main body (3) that is affixed bridging therebetween. The stomach-side sheet member (2A) and the back-side sheet member (2B) have: a main body section (20a, 20b) having a side sealing section (4) on the left and right sides； and an extension section (21a, 21b) that extends towards an under-groin section (C). The absorbent main body (3) has an absorbent body (33) and a lateral sheet section (34) positioned to the outside of both edges in the lengthwise direction of the absorbent body (33). At least at the under-groin section (C), the lateral sheet sections (34) have an expanding/contracting region (34c) exhibiting stretchability by means of an elastic member (36) affixed in an extended state. In the expanding/contracting region (34c), the length (Lm) from the center line (CL) of the article to the stomach-side end (34m) is longer than the length (Lu) from the center line (CL) to the back-side end (34u).
