A liquefied gas treatment system, comprising: a high pressure engine; a boil-off gas supply line connected from a liquefied gas storage tank to a high pressure gas engine; a boil-off gas compressor provided on the boil-off gas supply line and pressurizing to a pressure ranging between 200 bar and 400 bar boil-off gas generated from the liquefied gas storage tank; a boil-off gas heat exchanger provided upstream of the boil-off gas compressor on the boil-off gas supply line and exchanging heat between the boil-off gas, pressurized by the boil-off gas compressor and recovered along the boil-off gas supply line branching off upstream of the high pressure gas engine, and the boil-off gas supplied from the liquefied gas storage tank; a boil-off gas decompressor provided downstream of the boil-off gas heat exchanger on the basis of a flow of the boil-off gas, recovered along the boil-off gas supply line, and decompressing the boil-off gas compressed by the boil-off gas compressor; a vapor-liquid separator separating flash gas from the boil-off gas decompressed by the boil-off gas decompressor; a vapor recovery line connected from the vapor-liquid separator to the boil-off gas heat exchanger and supplying the flash gas to the boil-off gas heat exchanger; and a forcing vaporizer provided downstream of the liquefied gas storage tank and configured to vaporize the liquefied gas in the liquefied gas storage tank, supply the liquefied gas in gaseous state to the boil-off gas compressor and increase flow of the boil-off gas supplied to the high pressure gas engine; wherein the boil-off gas heat exchanger cools the boil-off gas, recovered along the boil-off gas supply line, with the boil-off gas, supplied from the liquefied gas storage tank, or the flash gas, supplied through the vapor recovery line, and wherein the boil-off gas decompressor is configured to decompress the boil-off gas to a pressure ranging from 1 bar to 10 bar. a high pressure engine; a boil-off gas supply line connected from a liquefied gas storage tank to a high pressure gas engine; a boil-off gas compressor provided on the boil-off gas supply line and pressurizing to a pressure ranging between 200 bar and 400 bar boil-off gas generated from the liquefied gas storage tank; a boil-off gas heat exchanger provided upstream of the boil-off gas compressor on the boil-off gas supply line and exchanging heat between the boil-off gas, pressurized by the boil-off gas compressor and recovered along the boil-off gas supply line branching off upstream of the high pressure gas engine, and the boil-off gas supplied from the liquefied gas storage tank; a boil-off gas decompressor provided downstream of the boil-off gas heat exchanger on the basis of a flow of the boil-off gas, recovered along the boil-off gas supply line, and decompressing the boil-off gas compressed by the boil-off gas compressor; a vapor-liquid separator separating flash gas from the boil-off gas decompressed by the boil-off gas decompressor; a vapor recovery line connected from the vapor-liquid separator to the boil-off gas heat exchanger and supplying the flash gas to the boil-off gas heat exchanger; and a forcing vaporizer provided downstream of the liquefied gas storage tank and configured to vaporize the liquefied gas in the liquefied gas storage tank, supply the liquefied gas in gaseous state to the boil-off gas compressor and increase flow of the boil-off gas supplied to the high pressure gas engine; wherein the boil-off gas heat exchanger cools the boil-off gas, recovered along the boil-off gas supply line, with the boil-off gas, supplied from the liquefied gas storage tank, or the flash gas, supplied through the vapor recovery line, and wherein the boil-off gas decompressor is configured to decompress the boil-off gas to a pressure ranging from 1 bar to 10 bar.