PROBLEM TO BE SOLVED: To provide an underpants type absorbent article facilitating discrimination between the front and back sides before being worn, and hardly leaving a mark on the groin region of a wearer when being worn.SOLUTION: An underpants type absorbent article 1 includes a belly side sheet member 2A, a back side sheet member 2B, and a vertically long absorbent body, and a pair of side seal parts 4 and 4, a waist opening part, and a pair of leg opening parts are formed by connecting both side edge parts of the belly side sheet member 2A with both side edge parts of the back sheet member 2B. The belly side sheet member 2A and the back side sheet member 2B include extension parts 21a and 21b extending downward from the side seal parts 4, respectively, and both of the extension parts 21a and 21b have an elastic property in an article cross direction Y. In the extension part 21a of the belly side sheet member 2A, parts 26 and 26 positioned on the outer sides of both side edges of the absorbent body 3 have a non-rectangular shape, and in the extension part 21b of the back side sheet member 2B, parts 27 and 27 positioned on the outer sides of both side edges of the absorbent body 3 have a rectangular shape.
