There is a method disclosed for detecting errors when supplying electric power to a heating device in an image forming apparatus that is capable of setting various options, has a coil in a heating roller, generates eddy-current in the heating roller by producing a high frequency magnetic field from the coil, and has a fixing device for fixing a developer image on a recording medium through the self-heating of the heating roller based on the eddy-current loss. This detecting method first judges comparative times to temperature arriving points at plural stages when executing the process to raise a temperature of the heating roller to a prescribed temperature based on the options that are set, applies prescribed electric power to the coil when the power source is turned ON, executes the process to raise a temperature of the heating roller to a prescribed temperature. In this process, temperatures of the heating roller are detected along the elapse of time and it is judged whether there is any error from the time elapse to the temperature arriving points at plural stages judged from the detected temperature and corresponding comparative times judged by the first judgment.