To provide a process and an apparatus for reliquefying boil-off gas(BOG) from liquefied natural gas(LNG) that has a low cost construction without need for a gas-liquid mixing process and efficiently performs reliquefaction of BOG by heat exchange with LNG. ! The process and the apparatus for reliquefying BOG from LNG comprises reliquefying BOG in an LNG storage tank 10 and returning it to the same storage tank 10. A part of LNG fed out of the LNG storage tank 10 is pre-evaporated in a sub-heat exchanger 16 to form natural gas(NG), which is then mixed with BOG. The mixed gas is heat exchanged with LNG in a main-heat exchanger 14 to thereby be condensed and the resultant liquid phase component is returned to the LNG storage tank 10. ! (C)2003,JPO