To efficiently reduce the content of a dissolved oxygen by performing the pressure boosting and the pressure reduction of a liquid containing the dissolved oxygen once or more, then mixing the liquid after the pressure boosting with an inert gas and separating the insert gas containing a dissipated oxygen from the liquid after the pressure reduction and exhausting the inert gas.

A liquid introduced into a liquid feed pump 1 through a valve 12 from a flow path 11 is pressurized to a preset pressure level using the liquid feed pump 1 and then is made to flow into liquid feed piping 2a in a gas introduction part 2 through a flow path 13, a valve 14 and a flow path 15. The liquid pressure is boosted by the pump 1 in such a manner that the difference between the primary side and the secondary side of the pump 1 side of a throttle part 3 is as specified. An inert gas introduced through valves 22, 23 from flow paths 21, 21 is adjusted to a specified pressure level using a pressure setter 23, and this pressure-adjusted inert gas is jetted into the liquid from a sparger 5 through an inert gas supply passage 6 to produce a fluid which is in a gas/liquid mixed state. The gas/liquid mixed fluid is fed into a tank 4 through a flow path 31 to be separated into a gas and a liquid after a dissolved oxygen is dissipated and then is loaded into inert gas bubbles, passing through the throttle part 3. After this process, the inert gas containing the oxygen is exhausted from an exhaust passage 8. Thus it is possible to remove the dissolved oxygen.