PURPOSE: To drive heat generating resistor varied in the area with a single power source by varying the shape of the heat generating resistor and first and second leads so that the resistance value between the first and second leads of the heat generating resistor is in inverse proportion to the area of the heat generating resistor.

CONSTITUTION: The shape of a heat generating resistor and first and second leads for those parts connected to the heat generating resistor is varied so that the resistance of the heat generating resistor is in inverse proportion. When a large heat generating resistor 142 quadruples a small heat generating resistor 141 in the area with respect to the width B and the length A of those parts connected to leads 131 and 151 of the small heat generating resistor 141, the large heat generating resistor 142 are equally divided into two heat generating resistor elements 142a and 142b and the width and the length of those parts connected to leads 132 and 152 of the two heat generating resistor elements are set at 2B and A respectively.