To prevent toner soiling of components, caused by a transfer material of defective fixing that is discharged carrying the unfixed toner image.

This device is composed so as to determine the temp. deference (S24) between the temp. (S21 and S23) of heating means (ceramic heater) right after a paper sheet (transfer material) entering into a fixing nip, and the temp. of the heating means right before the paper sheet pulled out of the fixing nip (S22). When the temp. difference is a specific value or layer, the image forming is immediately stopped, on judging that the heating means is out of order (S25). Since the operation can be stopped before the paper sheet is allowed to pass through the fixing device, the transfer material carrying the unfixed toner is prevented from being discharged.