In a heater according to the present invention, one of conductive lines is arranged to extend from a temperature detection element toward one end portion of a substrate in a lengthwise direction of the substrate, whereas the other the conductive line is arranged to extend from the temperature detection element toward the other end portion of the substrate in the lengthwise direction of the substrate, and at least one of the two conductive lines has an area that is inclined in both of the lengthwise direction and the widthwise direction of the substrate.