An agent for removing a halogen gas, such as chlorine, in a waste gas by means of reduction; a method for producing this agent; a method for removing a halogen gas by use of this agent; and a system for removing a halogen gas. The agent for removing the halogen gas contains at least pseudo-boehmite, that serves as a host material, and a sulfur-containing reducing agent, that serves as a guest material. 1-8% by weight of the reducing agent, in terms of elemental sulfur, based on the total amount of the pseudo-boehmite and sulfur-containing reducing agent is present in the agent. At least one inorganic compound selected from among oxides, carbonates salts and hydrocarbon salts of alkaline earth metal elements, transition metal elements and zinc group elements is additionally contained in the agent as a third component.