An elastic waist structure for a pair of trousers or skirt utilizing at least one elastic strip material which is always kept from sight. The strip material or materials stretch or contract behind a fabric cover or waist fabric. A tuck whose top edge is open is provided below the waist portion of the trousers or skirt, whose slidable inner fabric prevents formation of wrinkles or furrows outside the trousers or skirt. The tuck can be replaced by a pocket.