To establish a self-diagnosing system for an exhaust gas function part by utilizing a secondary air feeding means.

On an engine secondary air feeding device, a control means is provided with a target air-fuel ratio map during feed of secondary air. When an idle state is detected by a detecting signal from an operation state detecting means, secondary air is fed by a secondary air feeding means, and an air-fuel ratio is measured in a state that feedback learning control of a control means is varied to open control by a learned value. A function is provided to diagnose that the secondary air feeding means is abnormal when a measured air-fuel ratio is outside a given range.