A concentration of a gas introduced into a circulation blower and an exhaust blower is reduced with a simple apparatus configuration. In an exhaust system, exhaust gases from a plurality of drying chambers are joined to a joining duct to be then exhausted by an exhaust blower. A plurality of exhaust ducts having inlets at different positions in each drying chamber are provided, and the exhaust duct having the inlet in the lowest gas concentration area is used as a duct for an exhaust gas circulated to a circulation blower.