A reduction in the accuracy of a failure determination of a filter due to a reduction in the detection accuracy of a PM sensor is suppressed. In order to achieve this, provision is made for a filter that is arranged in an exhaust passage of an internal combustion engine for trapping particulate matter contained in an exhaust gas, an NOx selective reduction catalyst that is arranged at the downstream side of the filter and reduces NOx by means of a reducing agent which is supplied thereto, a supply device that supplies the reducing agent to the NOx selective reduction catalyst from the upstream side of the NOx selective reduction catalyst, a PM sensor that detects an amount of the particulate matter in the exhaust gas at the downstream side of the NOx selective reduction catalyst, and a prohibition part that prohibits the use of a detected value of the PM sensor in cases where the reducing agent supplied from said supply device passes through the NOx selective reduction catalyst.