A fixing device includes a heater including a heat generator that is divided into a plurality of heat generating portions in an axial direction of the endless belt. The heat generator defines a dividing region between adjacent ones of the plurality of heat generating portions and a non-dividing region other than the dividing region. A primary guide contacts and guides the endless belt. The primary guide is disposed opposite the dividing region of the heat generator and has a first thermal capacity. A secondary guide contacts and guides the endless belt. The secondary guide is disposed opposite the non-dividing region of the heat generator and has a second thermal capacity that is greater than the first thermal capacity.