Reducing agent supply error detecting method for an exhaust gas control catalyst and corresponding detecting apparatus

When a reducing agent is supplied from a supply valve into an exhaust port, an air-fuel ratio detected by an air-fuel ratio sensor becomes lower than an air-fuel ratio generated by fuel injected from a fuel injection valve into a combustion chamber. An estimated supply amount is obtained by periodically accumulating a reducing agent supply amount corresponding value which appears in the change in the air-fuel ratio, in step S186. Accordingly, the estimated supply amount can be obtained with high accuracy. Then, by comparing the estimated supply amount with a determination value in step S190, a supply error can be detected with high accuracy. As a result, an abnormal determination in step S194/a normal determination in step S196 concerning the supply valve can be performed accurately.