Abstract: A formalin gas counteracting and deodorizing device which is connected to a sterilization chamber including a main pipeline having an inlet for formalin gas at one end thereof and an outlet for processed gas at the other end, a fan connected to the inlet side midway in the main pipeline, an oxidation means for decomposition, a cooling means all connected in this order from the fan to the outlet, and a branch pipe provided between the oxidation means of the main pipeline and the cooling means. The process includes performing sterilization utilizing the sterilizer for a predetermined period of time, circulating the gas between the main pipeline, and the bypass pipe line for a predetermined period of time using the formalin gas counteracting and deodorizing device just before the end of the sterilization process, then circulating the gas between the sterilization chamber and the main pipeline for counteracting the formalin gas and introducing a counteractive into the sterilization chamber.