Disclosed herein are an inexpensive catalyst for removing nitrogen oxides effectively from an exhaust gas by either catalytic reduction or catalytic decomposition, and a process for removing nitrogen oxides from an exhaust gas by the aid of said catalyst. The invention will eliminate the disadvantages--high material cost and complex process for catalyst preparation--involved in the conventional technology. The catalyst contains as the major constituent a multiple oxide of the CaO--Fe.sub.x O type containing 5-50 wt % CaO, said catalyst reducing or decomposing nitrogen oxides. The process is designed for denitration of nitrogen oxides by the aid of said catalyst.