A cardboard box erector that folds four flaps provided on edges of an opening of a cardboard box so as to surround the opening is disclosed. The four flaps include a locking flap folded last among the four flaps. The cardboard box erector includes a folding unit configured to fold each of the four flaps in a predetermined order to close the opening, and a pushing unit configured to perform an action to push the locking flap inside the cardboard box and pull the locking flap back. The pushing unit has a suction unit configured to such and hold the locking flap.