An exhaust gas cleaning system, comprising: a reducing agent injection module installed in a housing which is configured to be mounted in an exhaust pipe through which an exhaust gas is discharged from an engine, the reducing agent injection module being configured to inject a reducing agent into the housing; a selective catalyst reduction device installed in rear of the reducing agent injection module and configured to catalytic react the reducing agent with nitrogen oxide included in the exhaust gas to reduce the nitrogen oxide; a differential pressure sensor to detect a differential pressure between a pressure in the housing in front of the selective catalyst reduction device and a pressure of the housing in rear of the selective catalyst reduction device; a first concentration sensor and a second concentration sensor respectively in front and rear of the selective catalytic reduction device to detect a concentration of nitrogen oxide within the housing; and a control device configured to receive the pressure information from the differential pressure sensor and concentration information of nitrogen oxide from the first and second concentration sensors (150, 152) and calculate conversion efficiency of nitrogen oxide of the selective catalytic reduction device from the received concentration information, and to output a first warning signal when the calculated conversion efficiency is lower than a predetermined efficiency and the detected differential pressure is lower than a predetermined pressure, a second warning signal different from the first warning signal when the calculated conversion efficiency is lower than the predetermined efficiency and the detected differential pressure is greater than a predetermined pressure and a third warning signal different from the first and second warning signals when the conversion efficiency is greater than the predetermined efficiency and the detected differential pressure is greater than the predetermined pressure. a reducing agent injection module installed in a housing which is configured to be mounted in an exhaust pipe through which an exhaust gas is discharged from an engine, the reducing agent injection module being configured to inject a reducing agent into the housing; a selective catalyst reduction device installed in rear of the reducing agent injection module and configured to catalytic react the reducing agent with nitrogen oxide included in the exhaust gas to reduce the nitrogen oxide; a differential pressure sensor to detect a differential pressure between a pressure in the housing in front of the selective catalyst reduction device and a pressure of the housing in rear of the selective catalyst reduction device; a first concentration sensor and a second concentration sensor respectively in front and rear of the selective catalytic reduction device to detect a concentration of nitrogen oxide within the housing; and a control device configured to receive the pressure information from the differential pressure sensor and concentration information of nitrogen oxide from the first and second concentration sensors (150, 152) and calculate conversion efficiency of nitrogen oxide of the selective catalytic reduction device from the received concentration information, and to output a first warning signal when the calculated conversion efficiency is lower than a predetermined efficiency and the detected differential pressure is lower than a predetermined pressure, a second warning signal different from the first warning signal when the calculated conversion efficiency is lower than the predetermined efficiency and the detected differential pressure is greater than a predetermined pressure and a third warning signal different from the first and second warning signals when the conversion efficiency is greater than the predetermined efficiency and the detected differential pressure is greater than the predetermined pressure.