An exhaust gas purifying apparatus for an internal combustion engine is provided. The apparatus includes a recoverable type exhaust gas purifying device provided in an exhaust passage, a reducing agent addition device for adding a reducing agent into the exhaust passage upstream of the exhaust gas purifying device, an operation control device for controlling operation of the reducing agent addition device such that the reducing agent is added into the exhaust passage in a recovering process of the exhaust gas purifying device, a pressure obtaining device for obtaining pressure in the exhaust passage downstream from the reducing agent addition device and upstream from the exhaust gas purifying device; and a diagnosing device for diagnosing the reducing agent addition device based on the pressure obtained by the pressure obtaining device at an estimate arrival-time when the reducing agent added from the reducing agent addition device is estimated to arrive at the exhaust gas purifying device in a case the operation of the reducing agent addition device is controlled by the operation control device such that the reducing agent is added into the exhaust passage.