PURPOSE: To suppress the upheaval and thermal deformation cycles of the center of glass projection and the breakage of exothermic elements by repeating heating pulses by providing a glass projection of a thermal expansion coefficient with the range of 30W35×10-7/°C for the lower part of heating elements on an insulating base plate.

CONSTITUTION: A heating element 3 is formed on the top of a trapezoidal projection 2 formed on an insulating alumina base plate 1. Conductors 4 are formed on the plate 1 and the projection 2 connectedly to both ends of the heating element 3 and led to the end portion through the upside of the base plate 1. The projection 2 is made of glass powder of a thermal expansion coefficient of 30W35×10-7/°C, having constituents, e.g., SiO2, Al2O3, B2O3 and Na2O, and a grain size distribution of 0.3W30μm with max. distribution of 6W10μm. The glass powder is melted at 950°C for 8min to form the projection 2. Since the thermal expansion coefficient of the projection 2 below the heating element 3 is small, thermal deformation of the glass projection 2 can be lessened even when the temperature of the heating element is raised by application of pulse voltage.