A bag supply apparatus for supplying bags onto a conveyer for transporting a bag bundle stacked such that an upper level bag is shifted forward, including a bag lifting device, which is provided with a pair of bag support members disposed facing one another on the left and right sides near the rear edge of a belt conveyor, and a rotation mechanism, which imparts translational motion of rotation in symmetric in a vertical plane with respect to the conveyance direction of the belt conveyor. The bag supporting portions of the bag support members contact the bag bundle from below to lift up the rear edge portion of the bag bundle and then separated from the bag bundle outwardly. While the bag supporting portions thus lifting the bag bundle, a new bag is fed into the space between the conveyance surface of the belt conveyor and the bag bundle.