PURPOSE: To always maintain the pressure inside a vacuum tank within a definite range and to heat and seal a semiconductor device without a defect by providing a limit sensor which detects the upper limit and the lower limit of the degree of vacuum inside the vacuum tank and which adjusts the evacuation air amount of a vacuum pump used to vacuum-evacuate the vacuum tank.

CONSTITUTION: The degree of vacuum in a vacuum tank 2 is made to reach a level of 5 by using a vacuum pump 3. When the degree of vacuum reaches the level of 5, the lower limit of a limit sensor is actuated. The evacuation capacity of the vacuum pump is reduced or stopped either by stopping the vacuum pump 3 or by shutting an opening and shutting valve. Then, a heater 1a is turned on; the heating and sealing operation of a semiconductor device as a half-finished product is started. After that, when a gas is generated from the semiconductor device 1 and the degree of vacuum is deteriorated, the upper limit of the limit sensor is actuated. The opening and shutting valve is opened or the vacuum pump is operated and its vacuum evacuation ability is displayed fully. Thereby, the degree of vacuum reaches the level of 5 again. The operation is repeated, the semiconductor device is heated and sealed for a prescribed time and this process is completed.