An image forming apparatus including: an image forming section to form a toner image on a recording sheet; a fixing device which has a fixing roller, an endless fixing belt trained with a tension about at least two rollers including the fixing roller, a heating device to heat the endless fixing belt, and a pressure roller to press the fixing roller through the endless belt, wherein the fixing device fixes the toner image on the recording sheet at a nip portion formed between the heated endless fixing belt and the pressure roller; a tension changing section to change the tension of the endless fixing belt; a recording sheet discriminating section to discriminate a type of recording sheet to be used; and a controller controling to make the tension changing section to change the tension of the fixing belt corresponding to the type of recording sheet.