The present invention is intended to provide a gas purification apparatus and a gas purification method with an excellent thermal efficiency and capable of degrading COS at a high degradation rate. A gas purification apparatus configured to purify gas at least including COS, H2O, CO2, and H2S includes a COS treatment device which is provided with a COS conversion catalyst and configured to treat and degrade COS in the gas by hydrolysis, and H2O adjustment means configured to adjust the concentration of H2O in the gas to be introduced into the COS treatment device.