The present invention intends to provide an agent and a method for removing harmful gas, which exhibits high harm-removing ability per unit volume for harmful halogen-containing gas contained in the exhaust gas from the etching or cleaning step in the manufacturing process of a semiconductor device, and which is inexpensive. The invention is characterized by that halogen-containing gas is removed using a harm-removing agent comprising a specific iron oxide, an alkaline earth metal compound and activated carbon in the specific amount. In the case where the exhaust gas contains halogen gas such as chlorine or a gas such as sulfur dioxide, the gas is rendered harmless by using in combination a harm-removing agent comprising activated carbon or zeolite.