A hydrogen purifying apparatus and method are provided for oxidizing and removing carbon monoxide (CO) in a modified gas containing CO in addition to a main component hydrogen gas. The apparatus and method use comprises a catalyst reaction segment having a catalyst layer for oxidizing CO, a material gas supplying segment for supplying the modified gas to the reaction segment via a material gas supply pathway, and an oxidant gas supplying segment for supplying an oxidant gas on the path of the material gas supply pathway. Preferably, the apparatus further comprises means for cooling the catalyst layer at the upstream side and means for heating the catalyst layer at the downstream side.