According to one embodiment, an image forming apparatus includes a heating section that heats an image on a sheet formed of decolorable color material; an image forming section that transfers the image formed of the decolorable color material to the sheet; a diagnosis image generating section that instructs the image forming section to generate a diagnosis image formed of a predetermined image on a sheet; a control section that controls the heating section to switch a fixing temperature for fixing the image formed of the decolorable color material on the sheet to a decoloring temperature for decoloring the image formed of the decolorable color material on the sheet; and a first transporting route that transports the sheet on which the diagnosis image of the decolorable color material is formed through the image forming section to the heating section maintained at the decoloring temperature.