A packaging bag with a zipper having a pair of male and female zipper elements is gripped at the laterally opposite side edges thereof with a pair of grippers. A pair of suction-adhering members are disposed at both sides of the packaging bag and actuated to adhere to the respective outsides of the male and female zipper elements of the bag. In this state, the suction-adhering members are moved away from each other, thereby disengaging the male and female zipper elements from each other. At this step, the suction-adhering member adhering to the outside of the male zipper element is pivoted to displace the suction-adhering portion thereof toward the mouth of the bag, thereby pulling the male zipper element obliquely upward.