An ignition timing controller for an engine includes basic knock-limit ignition timing calculation means that calculates a basic knock-limit ignition timing on the basis of an operating state of the engine; learning-region variation calculation means that learns the knock-limit ignition timing of the engine in two operating regions and thereby calculates a learning-region variation due to octane number and a learning-region variation due to humidity in one of the operating regions; estimated variation calculation means that estimates a variation due to octane number and a variation due to humidity in a present operating state on the basis of the learning-region variation due to octane number and the learning-region variation due to humidity; and knock-limit ignition timing calculation means that calculates the knock-limit ignition timing in the present operating state on the basis of the basic knock-limit ignition timing, the variation due to octane number, and the variation due to humidity.