A system which efficiently diminishes harmful substances without restricting the function of a gas source or combustion control. The system is characterized by comprising: a gas source (1) which discharges a gas whose composition and temperature fluctuate; a gas property controller (2) which is disposed downstream from the gas source (1) and which regulates the proportions of oxygen, nitrogen oxides, and carbon monoxide in the gas and the temperature of the gas so as to result in a given concentration ratio and a given temperature, respectively; and a catalyst (3) which is disposed downstream from the gas property controller (2) and which reduces the concentrations of carbon monoxide and nitrogen oxides through a first reaction in which carbon monoxide is oxidized and a second reaction in which nitrogen oxides are reduced with carbon monoxide.
