A fixing device, comprising a heating roller being rotatable and having a rotating circumferential surface to come in contact with a recording medium holding non-fixed developing agent images so as to heat and melt the non-fixed developing agent images and a temperature detecting unit having a temperature detecting element to detect temperature of the rotating circumferential surface and a support member to support the detecting element, wherein the support member is arranged to come in contact with the rotating circumferential surface at a first contact position on the rotating circumferential surface, the temperature detecting element is arranged to come in contact with the rotating circumferential surface at a second contact position which is located upstream of the first contact position in the rotating direction. This structure provides a high quality toner images by controlling fixing temperature based on a precise temperature detection of the heating roller.