An image forming apparatus includes a controller, a database, an image carrier, a transfer unit, a fixing device including a temperature detector and a first drive device, a speed changer, and an electric-current detector. The controller acquires write performance dependence on a variable representing each of multiple circumferential positions of the medium by measuring track average write performance with respect to the circumferential positions, and determines a second set of fitting coefficients to approximate by a periodic function the acquired dependence in terms of the first variable. In an OFDM system, this corresponds to a certain set of subcarriers in the timefrequency arrangement.