A filter abnormality determination system includes: a PM sensor (8) configured to introduce a part of exhaust gas flowing in the exhaust gas passage (2) on the downstream side of a filter (4) into a sensor body and to correlate a sensor output with an amount of particulate matter accumulated on a sensor element in the sensor body; an abnormality determining unit (20) configured to perform filter abnormality determination on the basis of the output of the PM sensor (8); a determination timing determining unit configured to determine a timing at which the abnormality determining unit (20) performs the abnormality determination on the basis of an estimated amount of particulate matter accumulated on the sensor element; and a determination performing adjusting unit configured to adjust the performing timing determined by the determination timing determining unit depending on a variation of an exhaust gas flow rate per unit time in the exhaust gas passage (2) in which the PM sensor (8) is disposed.
