Air is continuously supplied into upper spaces (9) via an air supply duct (22), during coating of an object to be coated in a paint booth (1), whereby the air is jetted through jetting holes (10) and flows downwards along side walls (1c, 1d). The air flowing downwards along the side walls is sucked into lower spaces (16), via clearances (13), and is discharged out of the paint booth via air exhaust ducts (24).