In order to suppress excessive temperature rise at a non-sheet-passing region while suppressing the deterioration of fixability in case of heating small size sheets, a heater includes a first heat-generation line and a second heat-generation line, which are independently controllable, wherein the first heat-generation line is configured so that a plurality of heat-generation resistors having positive resistance-temperature characteristics is electrically connected in parallel to each other between a first electro-conductive element and a second electro-conductive element, and the heat-generation resistors are adjusted so that a heat-generation amount per unit length of the first heat-generation line in a lengthwise direction of the heater decreases from a recording material conveyance reference towards a end portion in the lengthwise direction, and an image heating device having the heater.