PURPOSE: To automatically reduce the differential pressure by providing a pressure equalizing vessel having a pressure equalizing chamber communicating with a sealed chamber and varied in its volume at the lower low temperature part of a treating chamber.

CONSTITUTION: A sealed vessel 8 having a sealed chamber 7 is provided in a furnace chamber, a pressure equalizing vessel 10 having a pressure equalizing chamber 9 is provided at the low temperature part of the lower part of the furnace chamber, and the chamber 7 communicates with the chamber 9. If the pressure in the chamber 7 is larger than the pressure out of the chamber 7 during HIP treating, the volume of the chamber 9 is increased to lower the internal pressure in the chamber 7 and the pressure out of the chamber 7 is increased to reduce the differential pressure. On the other hand, if the pressure in the chamber 7 is smaller than the pressure out of the chamber 7, the volume of the chamber 9 is contracted to increase the internal pressure in the chamber 7 and the pressure out of the chamber 7 is lowered to reduce the differential pressure. Thus, the number of times of operations of a switching valve 17 can be reduced by the control of the differential pressure.