A controller for an internal combustion engine capable of performing ignition timing retard angle control for warming up a catalyst unit, and a number-of-revolutions feedback control by ignition timing for converging an engine rotation speed to a target rotation speed at the time of cold start, comprises a comparison means for comparing final ignition timing with retard angle lower limit during execution of the number-of-revolutions feedback control by ignition timing, and a means for altering the target rotation speed in the number-of-revolutions feedback control by ignition timing based on the comparison results from the comparison means.The target rotation speed is increased by a predetermined speed when the final ignition timing reaches the retard angle lower limit, and decreased by a predetermined speed when the difference between the final ignition timing and the retard angle lower limit exceeds a predetermined angle on an advance angle side. Consequently, an engine rotation speed can be converged to a target rotation speed quickly while a variation in rotation speed is suppressed at the time of cold start, and quick warm-up of the catalyst unit can be positively achieved.