Provided is an underwear-type diaper that that can be passed without difficulty around both legs opened in an M-shape of an infant less than half a year old. The underwear-type diaper (10) for an infant less than half a year old is such that both lateral edges of a front and a back waist panel (18, 19) provided with a waist elastic body are joined at a seam (20), a waist opening edge (22) is formed by the outer edges (18a, 19a) of the waist panels, and a pair of leg opening edges (23) are formed by both lateral edges (13c, 13d) of a crotch panel (13) and the inside edges (18b, 19b) of the front and back waist panels. When the connection of the seam is released and the underwear-type diaper (10) is extended in a flat shape, the dimensions (L1) of the spacing between the outer edges is in the range of 300-400 mm, and of the waist elastic bodies traversing the seam, the dimensions (L2) of the spacing between the waist elastic body at the front waist panel side closest to the inside edge of the front waist panel and the waist elastic body at the back waist panel side closest to the inside edge of the back waist panel is at least 55% of the dimensions of the spacing between the outer edges.
