A heater includes a substrate, a heating resistor layer, and an electrode layer. The substrate has a substrate obverse surface and a substrate reverse surface. The heating resistor layer is formed on the substrate obverse surface. The electrode layer is formed on the substrate obverse surface and in contact with the heating resistor layer. The electrode layer includes a pair of strip portions extending in a longitudinal direction of the substrate and spaced apart from each other in the width direction of the substrate. The heating resistor layer includes a main heating member and a sub heating member each extending in the longitudinal direction of the substrate and located between the strip portions in the width direction. The sub heating member has a higher temperature coefficient of resistance than that of the main heating member.