A method of heat-fixing a visible image of toner to a recording medium comprises applying a toner image onto the recording medium, whereinthe toner to form said toner image or the resin component of the toner has the properties such that the melt viscosity .eta.' measured by an overhead-type flow tester is from 10.sup.3 to 10.sup.6 poise at a temperature within the temperature range of from 120.degree. C. to 150.degree. C., and the absolute value of the inclination of a graph is not more than 0.50 ln (poise)/.degree.C. when the natural logarithms ln.eta. of the melt viscosities at 120.degree. C. and 150.degree. C. are plotted with respect to the temperatures; and heat-fixing the toner image retained on the recording medium to the recording medium by use of a heater element as stationarily supported and a pressure member that brings said recording medium into close contact with said heater element through a film interposed between them.