A fixing device includes a first member. The first member extends along a first longitudinal axis, and has an external surface whose thickness varies along the first longitudinal axis to define at least one first convex portion and at least one first concave portion. A thickness of the first convex portion is larger than a thickness of the first concave portion. A heating source is disposed in the first member. The heating temperature of an inner circumference side of the first convex portion is higher than the heating temperature of the inner circumference side of the first concave portion.