To automate a bagging operation for collected tray containers or the like.

A packing bag 1 whose rear sheet 1b is longer than a front sheet 1a and which is formed with a protruding piece 1c protruding toward a tip at a base of the rear sheet 1b is used. Thus, one of packing bags 1 positioned on an upper surface among numerous vertically stacked bags 1 is sucked and lifted. The lifted bag 1 is moved below a feed hopper in which objects A to be packed such as the tray containers are stored, its mouth is opened, and the objects A are put in the bag 1. Thereafter, the mouth is closed, and the closed mouth can be welded and sealed.