Abstract: A fixing device for a dry process copier selectively operable in a full-color copy mode and a black-and-white copy mode has a Teflon-coated first fixing roller which is a rigid roller, and a second fixing roller having an elastic layer thereon. In the full-color copy mode the second fixing roller fixes a toner image while, in the black-and-white copy mode, the first fixing roller fixes a toner image. When various units of the copier other than the fixing device are not activated, heat sources individually accommodated in the first and second fixing rollers are activated to warm up the fixing rollers rapidly to a predetermined temperature. While the full-color copy mode or the black-and-white copy mode is under way, either one of the heat sources which is to join in fixation is activated.