An exhaust gas cleaning system is provided that comprises a particulate filter and an NOx trapping catalytic converter disposed upstream of the particulate filter. The exhaust gas cleaning system is configured to determine a combustion amount of the particulate matter accumulated in the particulate filter that are combusted by the NOx in the exhaust gas, and adjust the combustion amount of the particulate matter based on an NOx adsorption condition of the NOx trapping catalytic converter. Then, the exhaust gas cleaning system is configured to estimate a particulate matter accumulation amount in the particulate filter by subtracting the adjusted combustion amount of the particulate matter from a discharge amount of the particulate matter. Thus, the amount of particulate matter accumulated in the particulate filter is estimated more accurately by taking into consideration the combustion of particulate matter by NOx in the exhaust gas.