To prevent condensation of components of low boiling points in air phase of a storage tank of low temperature liquefied gas in a closed cycle operation, and dispense with mixing adjustment operation by open-cycle operation by selectively removing components of low boiling points from BOG in a BOG piping.

BOG generated in a storage tank 1 is extracted from a piping 3 and boosted by a compressor 5 to 15 to 30kg/cm2. A pressure reducing valve 22 is so set as to operated with a proper value. All or a part of the boosted BOG is supplied to a separation device 21 through a bypass line 20. Components having boiling points lower than propane in BOG is adsosrbed by active carbon, or selectively separated by high polymer film due to remarkable difference of filtration velocity, and removed from a discharge pipe 23. Thus obtained BOG, containing only components of high boiling points, is liquefied, evaporated through pressure reduction, and fed back to the storage tank 1.