Sheets of paper discharged from an apparatus main body are stacked in a stack unit and humidified air is blown to the stacked sheets of paper by a spray device (humidifying seasoning). The humidified air blown by the spray device is blown between the sheets of paper to supply moisture to a non-image-formed portion. After blowing the humidified air for 5 minutes, nozzles of a water pipe is closed under the control of a controller and non-humidified air of the atmosphere is blown to the sheets of paper for 10 minutes (blowing seasoning). Accordingly, since the amount of moisture in the non-image-formed portion is returned to the amount of moisture originally contained in the sheets of paper, the difference in the amounts of moisture between an image-formed portion and a non-image-formed portion is reduced, thereby suppressing the cockling generated in the sheets of paper.