A diagnostic system for an internal combustion engine including a plurality of cylinder banks. The system includes a throttle position sensor mounted in each of the cylinder banks, for detecting a throttle opening of a throttle valve of an associated one of the cylinder banks. The throttle openings of the cylinder banks are controlled in accordance with a common target. An ECU makes a check to determine whether the detected throttle openings are same. The ECU determines in accordance with the comparison whether or not there is abnormality in at least one of the throttle valves and the throttle position sensors.