An exhaust emission control system for an internal combustion engine, the exhaust emission control system characterized by comprising: a filter arranged in an exhaust passage of the internal combustion engine, the filter configured to collect particulate matter contained in exhaust gas that is emitted from the internal combustion engine; a differential pressure sensor configured to detect a differential pressure between an upstream side and downstream side of the filter; and an electronic control unit including a first estimation portion and a second estimation portion in order to estimate an amount of accumulation of the particulate matter accumulated in the filter, the first estimation portion configured to estimate the amount of accumulation on the basis of an amount of particulate matter emitted from the internal combustion engine, the second estimation portion configured to estimate the amount of accumulation on the basis of the differential pressure between the upstream side and downstream side of the filter, the electronic control unit configured to: (a) estimate an amount of accumulation of particulate matter accumulated in the filter, (b) execute burning control for burning the particulate matter accumulated in the filter when the estimated amount of accumulation of the particulate matter has reached a preset threshold, (c) include a first threshold for the first estimation portion and a second threshold for the second estimation portion as the preset threshold, the second threshold being larger than the first threshold, and (d) allow the second estimation portion to estimate the amount of accumulation when an accumulated time, in which a condition for applying the differential pressure between the upstream side and downstream side of the filter is satisfied, is longer than or equal to a preset threshold. a filter arranged in an exhaust passage of the internal combustion engine, the filter configured to collect particulate matter contained in exhaust gas that is emitted from the internal combustion engine; a differential pressure sensor configured to detect a differential pressure between an upstream side and downstream side of the filter; and an electronic control unit including a first estimation portion and a second estimation portion in order to estimate an amount of accumulation of the particulate matter accumulated in the filter, the first estimation portion configured to estimate the amount of accumulation on the basis of an amount of particulate matter emitted from the internal combustion engine, the second estimation portion configured to estimate the amount of accumulation on the basis of the differential pressure between the upstream side and downstream side of the filter, the electronic control unit configured to: (a) estimate an amount of accumulation of particulate matter accumulated in the filter, (b) execute burning control for burning the particulate matter accumulated in the filter when the estimated amount of accumulation of the particulate matter has reached a preset threshold, (c) include a first threshold for the first estimation portion and a second threshold for the second estimation portion as the preset threshold, the second threshold being larger than the first threshold, and (d) allow the second estimation portion to estimate the amount of accumulation when an accumulated time, in which a condition for applying the differential pressure between the upstream side and downstream side of the filter is satisfied, is longer than or equal to a preset threshold. (a) estimate an amount of accumulation of particulate matter accumulated in the filter, (b) execute burning control for burning the particulate matter accumulated in the filter when the estimated amount of accumulation of the particulate matter has reached a preset threshold, (c) include a first threshold for the first estimation portion and a second threshold for the second estimation portion as the preset threshold, the second threshold being larger than the first threshold, and (d) allow the second estimation portion to estimate the amount of accumulation when an accumulated time, in which a condition for applying the differential pressure between the upstream side and downstream side of the filter is satisfied, is longer than or equal to a preset threshold.