An image forming apparatus includes a gradation converting circuit, a printer engine, an image inspecting circuit, and a color tone control circuit. The gradation converting circuit includes a threshold value generating section that generates threshold values, a first gradation correcting section that converts gradation values of image data, a second gradation correcting section that converts output values from the first gradation correcting section based on a gradation control parameter, and a gradation processing section that reduces tone levels of the second gradation correcting section by comparing output values from the second gradation correcting section and the threshold values. The color tone control circuit builds a prediction model for predicting, in response to the output values from the first gradation correcting section, measurement values to be measured by the image inspecting circuit, and corrects the gradation control parameter based on the measurement values and values predicted by e prediction model.