1. An image forming apparatus comprising: an image information obtainer to obtain image information; a gradation sequence processor to apply at least two different types of gradation sequence processing to the image information; a toner image formation device to form a toner image on a recording medium based on the image information; a fixing device to fix the toner image onto the recording medium in a fixing process; and a temperature controller to change a start-up temperature of the fixing device before the fixing process in accordance with inclusion of a halftone process and the type of gradation sequence processing applied to the image information.