A BOG reliquefaction apparatus for ships for transportation of liquefied gas, the BOG reliquefaction apparatus comprising: a multistage compressor (20a, 20b, 20c, 20d) comprising a plurality of compression stage parts and compressing BOG discharged from a storage tank storing liquefied gas; a plurality of cooling stage parts (21a, 21b, 21c, 21d) respectively disposed downstream of the plurality of compression stage parts (20a, 20b, 20c, 20d) and cooling the BOG compressed by the plurality of compression stage parts; a heat exchange unit cooling the BOG compressed by the multistage compressor (20a, 20b, 20c, 20d) through heat exchange; wherein the heat exchange unit comprises: a heat exchanger cooling the compressed BOG cooled by the cooling stage parts (21a, 21b, 21c, 21d) through heat exchange between the BOG cooled by the cooling stage parts (21a, 21b, 21c, 21d) and the BOG supplied from the storage tank to the multistage compressor (20a, 20b, 20c, 20d) and not subjected to compression, and a first expansion unit expanding some of the compressed BOG cooled by the heat exchanger; an intermediate cooler cooling the remaining compressed BOG cooled by the heat exchanger through heat exchange between the BOG expanded by the first expansion unit and the remaining cooled BOG; the BOG reliquefaction apparatus further comprising: a third expansion unit decompressing the remaining BOG cooled by the intermediate cooler, and a gas/liquid separator separating the BOG, which has been subjected to partial reliquefaction while passing through the third expansion unit; wherein the storage tank, the multistage compressor (20a, 20b, 20c, 20d), the heat exchange unit, the third expansion unit and the gas/liquid separator are provided to a reliquefaction line. a multistage compressor (20a, 20b, 20c, 20d) comprising a plurality of compression stage parts and compressing BOG discharged from a storage tank storing liquefied gas; a plurality of cooling stage parts (21a, 21b, 21c, 21d) respectively disposed downstream of the plurality of compression stage parts (20a, 20b, 20c, 20d) and cooling the BOG compressed by the plurality of compression stage parts; a heat exchange unit cooling the BOG compressed by the multistage compressor (20a, 20b, 20c, 20d) through heat exchange; wherein the heat exchange unit comprises: a heat exchanger cooling the compressed BOG cooled by the cooling stage parts (21a, 21b, 21c, 21d) through heat exchange between the BOG cooled by the cooling stage parts (21a, 21b, 21c, 21d) and the BOG supplied from the storage tank to the multistage compressor (20a, 20b, 20c, 20d) and not subjected to compression, and a first expansion unit expanding some of the compressed BOG cooled by the heat exchanger; an intermediate cooler cooling the remaining compressed BOG cooled by the heat exchanger through heat exchange between the BOG expanded by the first expansion unit and the remaining cooled BOG; the BOG reliquefaction apparatus further comprising: a third expansion unit decompressing the remaining BOG cooled by the intermediate cooler, and a gas/liquid separator separating the BOG, which has been subjected to partial reliquefaction while passing through the third expansion unit; wherein the storage tank, the multistage compressor (20a, 20b, 20c, 20d), the heat exchange unit, the third expansion unit and the gas/liquid separator are provided to a reliquefaction line. a heat exchanger cooling the compressed BOG cooled by the cooling stage parts (21a, 21b, 21c, 21d) through heat exchange between the BOG cooled by the cooling stage parts (21a, 21b, 21c, 21d) and the BOG supplied from the storage tank to the multistage compressor (20a, 20b, 20c, 20d) and not subjected to compression, and a first expansion unit expanding some of the compressed BOG cooled by the heat exchanger; an intermediate cooler cooling the remaining compressed BOG cooled by the heat exchanger through heat exchange between the BOG expanded by the first expansion unit and the remaining cooled BOG; the BOG reliquefaction apparatus further comprising: a third expansion unit decompressing the remaining BOG cooled by the intermediate cooler, and a gas/liquid separator separating the BOG, which has been subjected to partial reliquefaction while passing through the third expansion unit; wherein the storage tank, the multistage compressor (20a, 20b, 20c, 20d), the heat exchange unit, the third expansion unit and the gas/liquid separator are provided to a reliquefaction line. a third expansion unit decompressing the remaining BOG cooled by the intermediate cooler, and a gas/liquid separator separating the BOG, which has been subjected to partial reliquefaction while passing through the third expansion unit; wherein the storage tank, the multistage compressor (20a, 20b, 20c, 20d), the heat exchange unit, the third expansion unit and the gas/liquid separator are provided to a reliquefaction line.