A boil-off gas treatment method for a ship equipped with an engine supplied with a boil-off gas discharged from a storage tank storing liquefied gas, a compressor compressing the boil-off gas discharged from the storage tank and to be supplied to the engine, and an extra compressor installed in parallel with the compressor, provided for the case of failure of the compressor, comprising: branching the boil-off gas, which is discharged from the storage tank, into two to allow the compressor and the extra compressor to compress the boil-off gas of the branched two flows; sending at least one of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor to the engine as fuel; returning at least one of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor to the storage tank after cooling by heat exchanging with the boil off gas discharged from the storage tank and after re-liquefying by heat exchanging with a refrigerant made of an expanded boil-off gas; J and re-circulating at least one of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor to the upstream of the compressor and extra compressor to be joined, after being cooled by the refrigerant made of an expanded boil-off gas, being expanded to form the refrigerant made of an expanded boil-off gas and being used as the refrigerant, with the boil-off gas discharged from the storage tank after having cooled the returning boil-off gas; wherein the downstream line of the compressor and the downstream line of the extra compressor are connected to each other to join the boil-off gas compressed by the compressor with the boil-off gas compressed by the extra compressor, wherein the re-liquefaction amount can be increased by using boil-off gas compressed by the compressor and also boil-off gas compressed by the extra compressor as the recirculating boil-off gas. branching the boil-off gas, which is discharged from the storage tank, into two to allow the compressor and the extra compressor to compress the boil-off gas of the branched two flows; sending at least one of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor to the engine as fuel; returning at least one of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor to the storage tank after cooling by heat exchanging with the boil off gas discharged from the storage tank and after re-liquefying by heat exchanging with a refrigerant made of an expanded boil-off gas; J and re-circulating at least one of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor to the upstream of the compressor and extra compressor to be joined, after being cooled by the refrigerant made of an expanded boil-off gas, being expanded to form the refrigerant made of an expanded boil-off gas and being used as the refrigerant, with the boil-off gas discharged from the storage tank after having cooled the returning boil-off gas; wherein the downstream line of the compressor and the downstream line of the extra compressor are connected to each other to join the boil-off gas compressed by the compressor with the boil-off gas compressed by the extra compressor, wherein the re-liquefaction amount can be increased by using boil-off gas compressed by the compressor and also boil-off gas compressed by the extra compressor as the recirculating boil-off gas.