Fuel injection amount learning method for internal combustion engine

A learning method for an internal combustion engine (10) may include steps as follows. Injection for learning is executed from an injector (43A to 43D) during non-injection operation of the engine (10) with a timing. A correction amount for the injector (43A to 43D) is obtained based on an RPM fluctuation amount of the engine (10) due to the injection for learning and on the RPM of the engine (10). A controller (30) determines whether or not a learning control execution condition is satisfied with a predetermined condition with the timing. The controller (30) executes the learning control when the learning control execution condition is satisfied with the predetermined condition. The controller (30) obtains an execution frequency of the learning control. The controller (30) forcibly increases the execution frequency of the learning control when the execution frequency obtained in a previous step has not attained a predetermined frequency.