An exhaust gas-purifying system includes an exhaust gas-purifying unit that includes one or more exhaust gas-purifying catalysts, is supplied with the exhaust gas from a combustion engine, and purifies the exhaust gas so that the exhaust gas can be released to the atmosphere, a sensor that detects a concentration of nitrogen oxides contained in the purified exhaust gas, and a processing section that makes a judgment with respect to at least one of the exhaust gas-purifying catalysts, as to whether or not a performance is maintained at a sufficient level, based on the concentration detected by the sensor.