PURPOSE: To prevent transfer irregularity by uniformizing transfer pressure and to obtain a uniform image of good quality by processing a heat roller so as to form the central part of the core metal thereof into a domed shape and setting the outer diameter of the silicone rubber covering the core metal to the same dimension over the total length of the heat roller.

CONSTITUTION: The almost central part of at least one of the core metals 1BC, 2BC of a pair of heat rollers 1B, 2B is formed into a domed shape over the total length of the heat rollers. The outer diameter of the heat-resistant silicone rubber covering each of the core metals 1BC, 2BC is set to the same dimension over the total length of the heat rollers. Especially, when the max. outer diameter of the almost central part of each of the core metals 1BC, 2BC of the heat rollers is set to (a)mm and the outer diameter of each of both ends of the core metals of the heat rollers is set to (b)mm, a≥b+0.1mm is pref. formed. By this constitution, the heat rollers 1B, 2B are hard to generate a bowed change and the non-uniformity of pressure distribution is avoided and transfer irregularity becomes hard to generate and an image of good quality is obtained.