A heating roller includes: a core bar having a cylindrical shape; a rubber layer arranged on an outer circumferential surface of the core bar; and a release layer arranged on an outer circumferential surface of the rubber layer, wherein when thermally fixing a developer on a recording medium, the heating roller is heated to a temperature within a fixing temperature range including a minute particle-scattering start temperature at which minute particles start to scatter from the rubber layer. The heating roller is arranged inside a casing which is connected to a minute particle density measuring device; then the core bar of the heating roller inside the casing is heated to 230° C. by a heater; and when the density of the minute particles inside the casing is measured after elapse of 20 minutes since start of the heating, the density of the minute particles is less than 2,000 pieces/cm3.