Abstract: The lid of a box is pushed up from the main body of the box by injecting gas by use of one or more injection nozzles into a gap formed between the lid and main body of the box and then, with part of a top plate of the lid in contact with a lid rotating stopper, gas is further injected into the gap, or the lid is pushed up by injecting gas into the gap by use of one or more lower position injection nozzles and then gas is injected into the gap by use of one or more higher position injection nozzles in one or more stages, before the lid is opened from the main body. Thereby, it is possible to prevent the lids from being opened in a three-dimensional inclined state (in a one-side opened state) as well as to prevent the lid and main body of a box from being pushed upwardly simultaneously.