PURPOSE: To suppress the aging deterioration of a heater and improve the durability of the heater by reducing the local coagulation of platinum grains, and preventing the local concentration of a current owing to the density difference between the platinum grains.

CONSTITUTION: A platinum Pt paste is processed in the hydrogen H2 atmosphere at nearly 700-800°C, the melting point of the platinum is lowered to about 700°C when the platinum and hydrogen are reacted, and prescribed heater patterns 3, 4 are formed on green sheets 1, 2 with the low-melting point platinum paste. The heater patterns 3, 4 are dehydrogenated by the low-melting point platinum paste in the nonreducing atmosphere during or after the baking and degreasing of the green sheets 1, 2, and the melting point of the platinum is returned to about 1770°C. The platinum in the heater patterns 3, 4 is made nearly the complete metal crystal state when it is formed into a foil, and the local coagulation of platinum grains is reduced.