Disclosed is a system for re-liquefying a boil-off gas generated from a storage tank. A boil-off gas re-liquefying system comprises: a first compressor for compressing a part of a boil-off gas (hereafter referred to as "fluid A") that is discharged from a storage tank; a second compressor for compressing the other part of the boil-off gas (hereafter referred to as "fluid B") that is discharged from the storage tank; a second expanding means for expanding a part of a confluence flow (hereafter referred to as "fluid C") of fluid A and fluid B; a heat exchanger for cooling the other part of the confluence flow (hereafter referred to as "fluid D") of fluid A and fluid B; and a first expanding means for expanding fluid D that has been cooled by means of the heat exchanger, wherein the heat exchanger heat exchanges and cools fluid D by means of fluid C, which has been expanded by means of the second expanding means, as a refrigerant.