In an electrophotographic printing apparatus, a fixing device heat-roller control system configured for specially controlling the heat roller fixing temperature during consecutive dual-sided copying of a large number of sheets. The electrophotographic printing apparatus in particular is adapted for stack-less conveyance from the fixing unit of sheets onto one side of which the copy toner image has been fixed in dual-sided copying, switching the sheets back to image reproduction unit. The fixing unit has a heat roller incorporating an internal heater and a press roller in pressing contact with the heat roller. The fixing device heat-roller control system includes a sensor associated with the heat roller for detecting its superficial temperature, a switching element for supplying/cutting off voltage to the internal heater, and a temperature control unit connected to the sensor and the switching element. Based on detection output from the sensor, the temperature control unit controls the switching element to maintain target heating temperatures in the heat roller, such that in consecutive dual-sided printing a number of sheets, for every multiple of a predetermined printing sheet count, the target heating temperature in the heat roller lowers in predetermined stages to a predetermined temperature, below an initial target heating temperature for toner-image fixing sheets in single-sided printing.