Disclosed is a ship including a storage tank for storing a liquefied gas. The ship includes: a boil-off gas heat exchanger which is installed on a downstream of a storage tank and heat-exchanges a compressed boil-off gas (hereafter referred to as "a first fluid") by a boil-off gas discharged from the storage tank as a refrigerant, to cool the boil-off gas; a compressor which is installed on a downstream of the boil-off gas heat exchanger and compresses a part of the boil-off gas discharged from the storage tank; an extra compressor which is installed on a downstream of the boil-off gas heat exchanger and in parallel with the compressor and compresses the other part of the boil-off gas discharged from the storage tank; a boost compressor which is installed on an upstream of the boil-off gas heat exchanger and compresses the first fluid supplied to the boil-off gas heat exchanger; a refrigerant heat exchanger which additionally cools the first fluid which is cooled by the boil-off gas heat exchanger; a refrigerant decompressing device which expands a second fluid, which is sent to the refrigerant heat exchanger (a fluid sent to the refrigerant heat exchanger hereafter being referred to as "a second fluid") and cooled by the refrigerant heat exchanger, and then sends the second fluid back to the refrigerant heat exchanger; and a first decompressing device which expand the first fluid that is cooled by the boil-off gas heat exchanger and refrigerant heat exchanger, wherein the refrigerant heat exchanger heat exchanges and cools both the first fluid and second fluid by the boil-off gas, which has passed the refrigerant decompressing device, as a refrigerant, wherein the first fluid is either the boil-off gas which is compressed by the compressor or a confluent flow of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor, and the second fluid is either the boil-off gas which has been compressed by the extra compressor or a confluent flow of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor. Disclosed is a ship including a storage tank for storing a liquefied gas. The ship includes: a boil-off gas heat exchanger which is installed on a downstream of a storage tank and heat-exchanges and cools a compressed boil-off gas (hereafter referred to as 'a first fluid') by a boil-off gas discharged from the storage tank as a refrigerant; a compressor which is installed on a downstream of the boil-off gas heat exchanger and compresses a part of the boil-off gas discharged from the storage tank; an extra compressor which is installed on the downstream of the boil-off gas heat exchanger in parallel with the compressor and compresses the other part of the boil-off gas discharged from the storage tank; a refrigerant heat exchanger which additionally cools the first fluid which is cooled by the boil-off gas heat exchanger; a refrigerant decompressing device which expands a second fluid, which is sent to the refrigerant heat exchanger (a fluid sent to the refrigerant heat exchanger hereafter being referred to as 'a second fluid') and cooled by the refrigerant heat exchanger, and then sending same back to the refrigerant heat exchanger; a first decompressing device which is for expanding the first fluid that is cooled by the boil-off gas heat exchanger and refrigerant heat exchanger; and a third decompressing device which expands a part of the first fluid which is cooled by the boil-off gas heat exchanger (hereafter referred to as "a third fluid") and sends the expanded fluid back to the boil-off gas heat exchanger, wherein the boil-off gas heat exchanger uses the boil-off gas being discharged from the storage tank and the third fluid as a refrigerant and the refrigerant heat exchanger heat exchanges and cools both the first fluid and second fluid by the boil-off gas, which passes through the refrigerant decompressing device, as a refrigerant, wherein the first fluid is either the boil-off gas which is compressed by the compressor or a confluent flow of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor, and the second fluid is either the boil-off gas which is compressed by the extra compressor or a confluent flow of the boil-off gas compressed by the compressor and the boil-off gas compressed by the extra compressor.