Abstract: A secondary air pipe is connected on the upstream side from catalyst in an exhaust pipe, and a secondary air pump is provided at an upstream portion of the secondary air pipe. An opening/closing valve for opening/closing the secondary air pipe is provided on the downstream side from the secondary air pump. A pressure sensor for detecting pressure within pipe is provided between the secondary air pump and the opening/closing valve. An ECU calculates a secondary airflow rate based upon difference pressure between both secondary air supply pressure which is detected by the pressure sensor when the opening/closing valve is opened under such a condition that the secondary air pump is operated, and also, shutoff pressure which is detected by the pressure sensor when the opening/closing valve is closed.