An image forming apparatus capable of stabilizing image density on a long-term basis by compensating for an error in prediction of toner particle charge amount, while reducing variation in image density on a short-term basis by the prediction. A CPU predicts an amount of electrostatic charge of toner particles in a developer container and sets an exposure condition and causes a toner image to be fixed on a recording medium and a pattern image for detecting image density, to be each formed on a photosensitive drum according to the set exposure condition, while performing gradation correction based on the gradation conversion table. The CPU updates the gradation conversion table such that a detected density of the pattern image becomes closer to a target density. The CPU sets the exposure condition based on a result of the prediction such that toner image density becomes constant.