A fixing device may include: an endless belt circularly movable about a first axis extending in an axial direction; a nip member and a heater extending in an internal space defined by an inner peripheral surface of the endless belt; and a rotary body disposed to be opposite to the heater with respect to the nip member. The nip member may slidingly contact the inner peripheral surface. The rotary body and the nip member may nip the endless belt therebetween. The nip member may include a main portion extending in the axial direction and having a first axial end portion; and a protruding portion protruding from the first axial end portion toward the heater, the protruding portion having a first surface configured to receive radiant heat from the heater.