The apparatus heat fixes a toner image on one surface of a wet-developed, image-carrying copy sheet while maintaining the opposite surface of the sheet in sliding contact with the surface of a heated plate. A portion of the heating surface is formed with a concave surface, or a curved surface having a positive curvature, and there is provided a guide roller for introducing the sheet onto the curved surface. When introduced onto the curved surface, the sheet achieves a good sliding contact with the heating surface by virtue of its resiliency without application of a back pressure thereto, thus accomplishing an efficient heat fixing without any accompanying degradation in the image quality.