A method for manufacturing a rectangular-bottomed packaging bag includes an unfolding step of unfolding a flat bag by separating a pair of opening/closing plates (13) inserted into the flat bag from each other in a thickness direction of the flat bag, and a gusset forming step, after the unfolding step, of forming a side gusset and a bottom gusset by thrusting folding plates (11 and 12) between the pair of opening/closing plates (13) from the outside of the bag toward the inside of the bag.