A planar heat generating resistor has a heat generating resistor layer formed on or above a support member and a pair of opposing electrodes formed on the heat generating resistor layer, such that a width of the heat generating layer at the electrode area is larger than a width of the electrodes and a voltage is applied across the electrodes, in which a ratio of a maximum value of a gradient of .phi., .sqroot.(.differential..phi./.differential.x).sup.2 +(.differential..phi./.differential.y).sup.2 to a value of .sqroot.(.differential..phi./.differential.x).sup.2 +(.differential..phi./.differential.y).sup.2 at a center of the resistor is no larger than 1.4 when a Laplace equation .differential..sup.2 /.differential.x.sup.2 +.differential..sup.2 .phi./.differential.y.sup.2 =0 is solved for the heat generating resistor when an orthogonal coordinate system X-Y is defined on the resistor surface, a potential at a point (x,y) on the resistor surface is represented by .phi.(x,y), a boundary value is imparted to an area of a circumferential boundary of the resistor which contacts to one of the electrodes, a different boundary value is imparted to an area which contacts to the other electrode, and a boundary condition in which a differential coefficient of .phi. to a normal direction of the circumferential boundary is zero is imparted to an area which does not contact to any of the electrodes.