A fixing device includes a first member and a second member. The first member extends along a first longitudinal axis, and has a first elastic layer whose thickness varies along the first longitudinal axis to define at least one first convex portion and at least one first concave portion. The second member extends along a second longitudinal axis parallel to the first longitudinal axis, and has a second elastic layer whose thickness varies along the second longitudinal axis to define at least one second convex portion and at least one second concave portion. At least one of the first and second members is heated, and at least one of the first and second members is pressed against the other, with the first convex portion engaging the second concave portion and the first concave portion engaging the second convex portion, to define a fixing nip therebetween.