With a conventional individually packaged wet sheet, face mask, or other sheet, not only the fingertips grasping the sheet but also other fingers and the palm and back of the hand get dirtied by the cosmetic material, etc., as the fingers are inserted into the flat bag. The present invention aims to solve this problem, while simplifying the operation of spreading the folded-up sheet that has been removed from the flat bag and thereby allowing the removed sheet to be attached on the face quickly and reliably without dirtying the fingers. To this end, a flat bag storing a liquid-impregnated sheet-shaped object is provided that has on its top side a joined part constituted by two films sealed together, wherein the sealed part forming the joined part is formed at the base of the joined part over the entire length of the joined part, the edge of the joined part is not sealed but forms a non-sealed part instead, and the end edge of one of the two films constituting the non-sealed part is formed longer than the end edge of the other film.