A heater wherein an elongated substrate, a first electroconductive member, a second electroconductive member, a plurality of heat generating resistors, and a temperature detecting element. The following relationships are satisfied: W≥L and W≥S, where W represents a dimension of the temperature detecting element measured in a longitudinal direction of the substrate, L represents a dimension, measured in the longitudinal direction, of one of the heat generating resistors at least partially overlapping with the temperature detecting element with respect to the longitudinal direction, and S represents a dimension between adjacent heat generating elements of the heat generating elements.