An unfolding type disposable diaper (1) for infants weighing 3000 grams or less at birth said diaper (1) having leg surrounding elastic members (21) for constricting leg surrounding openings (HL) in the longitudinal direction the leg surrounding elastic members (21) each being provided in the longitudinal direction in positions on both sides in the transverse direction in a crotch section (1m). A back side section (1b) has a waist surrounding elastic sheet (31) for constricting a waist surrounding opening (HB) in the transverse direction the waist surrounding elastic sheet (31) being provided so as to extend to both sides in the transverse direction. When the diaper (1) is unfolded end sections in the transverse direction of the waist surrounding elastic sheet (31) are positioned with respect to the transverse direction at the same positions as end sections on the back side in the longitudinal direction of the leg surrounding elastic members (21) nearest thereto or are positioned further outside in the transverse direction relative to the end sections of the leg surrounding elastic members (21) nearest thereto and the waist surrounding elastic sheet (31) is disposed astride longitudinal center positions (C11m) of engaging sections (11m) of fastening tapes (11) in the longitudinal direction.
