A fixing unit for an image forming apparatus passes a recording medium with a unfixed toner image through between a pair of rotating bodies to fix the unfixed image to the recording medium. The fixing unit comprises a first heater for heating up one of the rotating bodies, a second heater for heating up the other of the rotating body, and a switching means for changing over a first state that the first heater and the second heater are serially connected to a power source and a second state that the first heater is connected to the power source, with the second heater being disconnected from the power source, wherein the relation: Q.sub.11/Q.sub.21>1/8 is satisfied, where Q.sub.11 denotes a heating value of the first heater in the first state per unit of time and Q.sub.21 denotes a heating value of the first heater in the second state per unit of time.