The present invention relates to a process for producing ultra-pure water, an apparatus for producing said ultra-pure water and a process for using the ultra-pure water produced according to said process. More particularly, the present invention relates to a process for producing ultra-pure water which comprises boiling a raw water to vaporize off the volatile components from the raw water, subsequently generating steam from the water, contacting the steam with a hydrophobic, porous, gas-permeable and liquid-impermeable membrane to make the steam permeate the membrane, and then condensing the steam, as well as to an apparatus for producing said ultra-pure water and a process for using said ultra-pure water.