A bag supply device receives large bags are retained in a stacked state on its mounting portions that are provided on chains. In addition, this bag supply device includes a horizontal support mechanism that supports the rear portion of the stacked large bags so that they can be raised and lowered in order to retain an uppermost surface of an uppermost large bag in a horizontal posture at a discharge position. The chains and the mounting portions are disposed such that the rear portion of the large bags hangs down. The horizontal support mechanism includes a roller that moves along the transport direction, and the uppermost large bag will be retained in a horizontal posture by moving the roller to support the rear portion of the large bags from below.