INTERNAL COMBUSTION ENGINE DIAGNOSTIC METHOD AND INTERNAL COMBUSTION ENGINE DIAGNOSTIC DEVICE

A prescribed air-fuel ratio feedback control region for performing feedback-control of air-fuel ratio has therein: a first region where fuel is injected, during one combustion cycle, only from a first fuel injection valve injecting fuel directly into a cylinder; and a second region where fuel is injected, during one combustion cycle, from both of the first fuel injection valve and a second fuel injection valve injecting fuel into an air-intake passage. A second region is configured such that an amount of fuel injected from the first fuel injection valve and an amount of fuel injected from the second fuel injection valve remain at a given constant ratio regardless of operating status. A diagnosis on the first and second fuel injection valves is performed by using a first air-fuel ratio learning value learned in the first region and a second air-fuel ratio learning value learned in the second region.