A processing unit for a substrate comprises a partition 6 provided between an atmospheric area S1 and an inert gas area S2. The partition 6 has an opening 22 to communicate the atmospheric area S1 and the inert gas area S2. A door 23 is provided at the opening 22 to open and close it. Carrier holding device 11 is provided for holding a carrier 2 of the substrate in such a manner that the carrier 2 comes in contact with the opening 22 on the side of the atmospheric area. Inert gas replacing device 56 is provided for replacing a gas in the carrier 2 with an inert gas by introducing the inert gas into the carrier 2 when the door 23 closes the opening 22 and the carrier 2 comes in contact with the opening 22 on the side of the atmospheric area by the carrier holding device 11. This processing unit can perform the process of the substrate without increasing the concentration of the oxygen in the inert gas area S2 by preventing leakage of the air from the atmospheric area S1 into the inert gas area S2.