In a device that opens the mouth of each one of the bags that are suspended with the mouths facing upward and conveyed in a vertical state at a uniform speed and uniform spacing in a straight line, a pair of suction disks revolve continuously in mutually opposite directions on circular tracks on the horizontal plane so that the suction-chucking surfaces of the suction disks are always caused to face each other. The suction disks revolve at a constant speed while maintaining substantially symmetrical positions relative to each other and revolve at the same speed as the speed the bags are conveyed.