An exhaust pipe is provided with a SCR catalyst (NOx catalyst) and an oxidation catalyst. A urea water adding valve and an exhaust gas temperature sensor are provided upstream of the SCR catalyst. A downstream NOx sensor is provided downstream of the SCR catalyst. The oxidation catalyst is provided with a catalyst temperature sensor. An ECU computes a temperature of the oxidation catalyst based on a detection value of the catalyst temperature sensor. Further, the ECU computes a temperature of exhaust gas flowing into the oxidation catalyst based on a detection value of the exhaust gas temperature sensor. The ECU detects ammonia flowing out from the SCR catalyst based on a differential temperature between the temperature of the catalyst and the temperature of the exhaust gas.