An object of the present invention is to provide a technology that enables detection of a fault of a PM trapper with improved accuracy. In the present invention, a pressure sensor 20 that measures a differential pressure across a PM trapper 11 is provided, and a fault of a PM trapper is determined based on an amount of change in the differential pressure across the PM trapper 11 between two different operation states of an internal combustion engine 1 in which the ambient temperature around the pressure sensor 20 falls within a specific temperature range. Since variations in the offset error of the differential pressure sensor 20 caused by changes in the ambient temperature around the differential pressure sensor 20 is made small, the amount of change in the differential pressure across the PM trapper 11 can be measured more accurately, and a fault of the PM trapper 11 can be detected with improved accuracy.