Air is taken out from a drying oven 1 for drying a coating film of a work piece 2, and the air is cooled such that each of at least part of moisture and at least part of a VOC which are contained in the air is condensed to be removed from the air. The air after the cooling is heated, and is returned into the drying oven 1. A heat pump 3 whose heat absorption source is the air taken out from the drying oven 1 and whose heat radiation source is the air after the cooling is provided. By using the heat pump 3, cooling and heating of the air are performed.