A fixing device included in an image forming apparatus for fixing a toner image formed on a sheet of the present invention includes a rotatable member and a pressing member forming a nip, which corresponds to a conveying zone, therebetween. The rotatable member and pressing member convey the sheet via the nip for thereby fixing the toner image on the sheet. A heating member heats the surface of the rotatable member in contact with therewith. Part of the rotatable member and part of the heating member contacting each other respectively have a convex shape and a concave shape complementary to the convex shape.