PURPOSE: To provide an apparatus for holding pressure constantly without needing an external pressure adjusting circuit.

CONSTITUTION: An apparatus for holding constantly pressure is provided with a cylindrical tank 2 divided by a partition part 7 into a lower part 6 including cold water, an upper part 5 including hot water and a large amount of steam 9 balanced above the hot water. The apparatus for holding constantly pressure interconnects the first and second parts and further comprises at least one of ducts 10, 11 having second ends 10a, 11a immersed in hot water in the upper part 5 and second ends 10b, 11b immersed in cold water in the lower part 6 and at least one condensing duct 12 having a first end 12a disposed in the steam portion 9 and a second end 12b immersed in cold water in the lower part 6.