A system for reliquefying a boil off gas generated in a storage tank includes a first compressor compressing a partial amount (hereinafter, referred to as ‘fluid a’) of boil off gas discharged from the storage tank, a second compressor compressing another partial amount (hereinafter, referred to as ‘fluid b’) of boil off gas discharged from the storage tank, a second expanding unit expanding a partial amount (hereinafter, referred to as ‘fluid c’) of a flow formed as the fluid a and the fluid b join, a heat-exchanger cooling another partial amount (hereinafter, referred to as ‘fluid d’) of the flow formed as the fluid a and the fluid b join, and a first expanding unit expanding the fluid d cooled by the heat-exchanger, wherein the heat-exchanger heat-exchanges the fluid d with the fluid c as a coolant expanded by the second expanding unit to cool the fluid d.