A heat fixing device capable of preventing deterioration of a temperature sensor by measuring the surface temperature of a heating roller at all times, and an image forming apparatus. The heat fixing device includes a temperature measurement unit for measuring the surface temperature of the heating roller. The temperature measurement unit further includes the temperature sensor, a conduit having a reflective surface formed inside thereof to lead infrared rays radiating from the heating roller to the temperature sensor, and a thermal insulation member installed between the conduit and the temperature sensor.