A heat roller fixing device for use in an electrophotographic copying apparatus and the like, which includes a heating roller constituted by laminating a fluorine resin layer on an electrically conductive core member through a primer layer and driven for rotation, and a pressure roller constituted by forming an electrically insulative layer on another electrically conductive core member and held in contact with the heating roller for simultaneous rotation with the heating roller so as to fix a positively charged toner image formed on copy paper onto the copy paper by causing the copy paper to pass between the heating roller and the pressure roller. The primer layer is composed of an offset prevention agent containing carbon black by a predetermined amount, preferably 0.4 to 3.0% by weight.