A ship comprising an engine, the ship further comprising: a storage tank storing liquefied natural gas; a self-heat exchanger configured to perform heat exchange with respect to boil-off gas discharged from the storage tank; a generator and/or a low-pressure engine using natural gas as fuel; a multistage compressor configured to compress the BOG discharged from the storage tank and having passed through the self-heat exchanger in multiple stages; the ship being configured such that the compressed BOG is at least partially sent to the self-heat exchanger and optionally partially to the generator and/or the low-pressure engine; wherein the self-heat exchanger is configured to cool the at least a part of the BOG compressed by the multistage compressor using the BOG discharged from the storage tank, a liquefied natural gas separated by a first gas/liquid separator, and a gaseous BOG separated by the first gas/liquid separator as a refrigerant; a first decompressor configured to expand some of the BOG compressed by the multistage compressor and cooled in the self-heat exchanger; a second decompressor configured to expand the other BOG compressed by the multistage compressor and cooled in the self-heat exchanger, the first gas/liquid separator configured to separate the gaseous BOG and the liquefied natural gas generated by partial reliquefaction through compression by the multistage compressor, cooling by the self-heat exchanger, and expansion by the first decompressor, wherein the ship is configured such that the liquefied natural gas and the gaseous BOG separated by the first gas/liquid separator and having passed through the self-heat exchanger as the refrigerant is sent to at least one of the generator and the low-pressure engine. a storage tank storing liquefied natural gas; a self-heat exchanger configured to perform heat exchange with respect to boil-off gas discharged from the storage tank; a generator and/or a low-pressure engine using natural gas as fuel; a multistage compressor configured to compress the BOG discharged from the storage tank and having passed through the self-heat exchanger in multiple stages; the ship being configured such that the compressed BOG is at least partially sent to the self-heat exchanger and optionally partially to the generator and/or the low-pressure engine; wherein the self-heat exchanger is configured to cool the at least a part of the BOG compressed by the multistage compressor using the BOG discharged from the storage tank, a liquefied natural gas separated by a first gas/liquid separator, and a gaseous BOG separated by the first gas/liquid separator as a refrigerant; a first decompressor configured to expand some of the BOG compressed by the multistage compressor and cooled in the self-heat exchanger; a second decompressor configured to expand the other BOG compressed by the multistage compressor and cooled in the self-heat exchanger, the first gas/liquid separator configured to separate the gaseous BOG and the liquefied natural gas generated by partial reliquefaction through compression by the multistage compressor, cooling by the self-heat exchanger, and expansion by the first decompressor, wherein the ship is configured such that the liquefied natural gas and the gaseous BOG separated by the first gas/liquid separator and having passed through the self-heat exchanger as the refrigerant is sent to at least one of the generator and the low-pressure engine.