Abstract: The present invention is directed to a harmful gas removing agent used for removing harmful gas such as CO, NO.sub.x, and O.sub.3 from exhaust gases ventilated from automobile tunnels and underground parking, thereby preventing air pollution. The harmful gas removing agent, solving the problems of the conventional adsorbents and catalysts, is capable of efficiently removing NO.sub.x with no need of pre-treatment of adding O.sub.3 to the exhaust gas being treated, as well as being capable of satisfactorily removing CO and O.sub.3 from exhaust gases even at high humidity, and its removing activity lasts for a long time. Furthermore, the harmful gas removing agent can be regenerated by being exposed to hot air at relatively low temperature, thereby being repeatedly used, and is hard to be poisoned by SO.sub.x contained in the exhaust gas.