Power is supplied from a capacitor of an auxiliary power source to a secondary heating element of a heating unit, thereby preventing temperature drop of a fixing roller at the time of continuous paper delivery. An allowable range for starting discharge from the capacitor is set to, for example, 30 to 40 volts. A discharge time in which the voltage of the capacitor reduces to a termination value, such as 20 volts, is longer than a time necessary for a reading unit to read a maximum number (for example, 100 sheets) of documents that can be read at one time.