The temperature characteristics of heating with an electric power source were stable. The heating temperature reached approximately 1300 °C with an electric power of approximately 300 W. In addition, the heating temperature could be controlled well by adjusting the electric power. The pressure increased owing to gas generated from the heaters and then decreased with increasing heating temperature. However, the pressure began to increase again at approximately 1000 °C. The reason for the increase in temperature is mainly the gas generated from the vacuum chamber. This is confirmed by the dominance of the increase in temperature of the vacuum chamber owing to radiant heat at high temperature and by a comparison of the pressure change during the heating and cooling processes.