A particulate accumulation amount estimating system is configured to accurately estimate an amount of particulate matter accumulated in a particulate filter based on the pressure difference across the particulate filter even when ash is accumulated in the particulate filter. The particulate accumulation amount estimating system is configured to estimate a particulate accumulation amount of particulate matter accumulated in the particulate filter based on a pressure difference that was detected across the particulate filter and an effective area change of the particulate filter that was detected due to the accumulation of ash in the particulate filter. The estimate of the particulate accumulation amount based on the pressure difference across the filter will be lower as the estimate value of the ash accumulation amount becomes larger.