An apparatus for treating a substrate which includes a chamber and an opening formed in the chamber allowing the substrate to be conveyed into the chamber or taken out thereof. The chamber, also, includes a detachable baffle plate that fits around an electrode. For treatment to commence, the substrate is placed on the electrode and the chamber is exhausted of or supplied with gases. The electrode is then vertically lifted together with the baffle plate and the baffle plate is moved either to a position that is higher in level than an upper end of the opening of the chamber or to a position that is lower in level than a lower end of the opening of the chamber. This allows the baffle plate to shield a region near the opening of the chamber from a treatment region and allows reaction products to be adhered to the baffle plate.