An underwear type diaper provided with: a diaper body (2) having a front section (20) for covering a wearer s ventral side a back section (21) for covering a wearer s dorsal side and a crotch section (22) between the front section (20) and the back section (21) one side panel (3) overlapping in a waist encircling direction with one side edge of the front section (20) and with one side edge of the back section (21) and a different side panel (3) overlapping in the waist encircling direction with a different side edge of the front section (20) separated in the waist encircling direction from the one side edge thereof and with a different side edge of the back section (21) separated in the waist encircling direction from the one side edge thereof wherein the side panels (3) are seamlessly continuous in the waist encircling direction and contain a filamentous elastic member (F) extending in the waist encircling direction and a pair of webbings (W) sandwiching the elastic member.
