A device for detecting abnormality of the fuel supply system of an internal combustion engine inhibits supply of evaporative fuel to the intake passage when the air-fuel ratio correction coefficient becomes smaller than a predetermined value, and thereafter determines that the fuel supply system is normal when the air-fuel ratio correction coefficient increases above the predetermined value as a result of the inhibition of the supply of the evaporative fuel. The supply of the evaporative fuel to the intake passage is resumed after the determination that the fuel supply system is normal has been made. Determination of abnormality of the fuel supply system is inhibited after resuming of the supply of the evaporative fuel until the engine enters a predetermined operating condition in which the air-fuel ratio correction coefficient has increased. The determination of abnormality of the fuel supply system is permitted to be resumed, based on the air-fuel ratio correction coefficient when the engine has entered the predetermined operating condition.