A retainer with a bottomed retaining hole and a pair of insertion grooves so that a self-standing packaging bag in a closed state is set in the retaining hole and insertion grooves and then the bag is opened and kept opened during packaging operations. The retaining hole that is vertically formed accommodates the body portion of the self-standing packaging bag, and both side edges of the bag are inserted in the insertion grooves which are vertically installed in symmetrical positions on both sides of the retaining hole. The depth of the bottomed retaining hole and insertion grooves is set to be smaller than the length of the self-standing packaging bag, the spacing between the end portions of the insertion grooves is set to be equal to or greater than the width of an unopened self-standing packaging bag, and the angle between the inside surfaces of the retaining hole and insertion grooves is set at or about 90 degrees.