A planar heat generating element includes an insulating layer and a resistance heat generator formed on one surface in a thickness direction of the insulating layer, for generating heat under a passage of electric current therethrough, the resistance heat generator constituting a plane of certain configuration as a whole and forming a heat generating pattern. The resistance heat generator includes a plurality of linear portions formed on one surface of the insulating layer that extend transversely with respect to a longitudinal direction of the insulating layer, the linear portions being arranged side by side substantially in parallel to each other; and a low-volume resistivity portion formed on the one surface so as to extend in the longitudinal direction of the insulating layer, and connecting the adjacent linear portions at their extending direction-wise ends to form a single line composed of the adjacent linear portions and the low-volume resistivity portion.