A controller for internal combustion engine includes a rotation number detecting section to detect engine rotation number and an air-fuel ratio detecting section to detect an air-fuel ratio (A/F) of air-fuel mixture and is configured to perform leaning control of the A/F. The controller further includes a rotation number difference calculating section to calculate a difference between a rotation number variation value of a worst cylinder specified as having largest rotation number variation based on the detected rotation number detected and an average value of rotation number variation values of remaining cylinders other than the worst cylinder; an estimated imbalance rate calculating section to calculate an estimated imbalance rate based on the A/F detected by the air-fuel ratio detecting section; and a lean abnormality detecting section to detect lean imbalance abnormality in a specific cylinder based on the calculated rotation number difference and the calculated estimated imbalance rate.