A vacuum chamber-forming method for forming a vacuum chamber in a power element of a control valve for a variable capacity compressor through a reduced number of steps. A power element is assembled in the atmospheric air by arranging a disk, a diaphragm, a disk, a spring and an upper housing on a lower housing, caulking the periphery of the lower housing to the periphery of the upper housing, and then soldering the junction of the upper and lower housings. The assembled power element is placed in a vacuum container, and a small hole formed in the upper housing is subjected to spot welding in the vacuum atmosphere, whereby the small hole is sealed by a weld metal.