A heater is to face a pressure element via a belt and is to heat the belt. The pressure element sandwiches and conveys a sheet together with the belt. The heater includes a substrate, a heat generating element, and a protective layer. The heat generating element is on the substrate. The protective layer covers the heat generating element and the substrate and whose surface is concave with respect to the pressure element. In the protective layer, a curvature of a top surface of an upstream end in a conveyance direction of the sheet is smaller than a curvature of a top surface of a downstream end in the conveyance direction.