A bunkering ship for bunkering liquefied fuel gas to a liquefied-fuel-gas propulsion ship on the sea, wherein the bunkering ship comprises: a liquefied-fuel-gas storage tank installed in the bunkering ship ; a fuel supply pipe configured to be connected to the liquefied-fuel-gas storage tank and to supply fuel to the liquefied-fuel-gas propulsion ship ; and a boil-off gas (BOG) collection pipe configured to be connected to the liquefied-fuel-gas propulsion ship and to collect BOG produced in a liquefied fuel gas tank of the liquefied-fuel-gas propulsion ship while the liquefied-fuel-gas is supplied; wherein the bunkering ship further comprises a BOG treatment unit, wherein the BOG treatment unit is configured to treat the BOG collected through the BOG collection pipe , wherein the BOG treatment unit comprises a high-pressure compressor configured to compress the BOG collected through the BOG collection pipe to high pressure and a low-pressure compressor configured to compress the BOG collected through the BOG collection pipe to low pressure, wherein the BOG treatment unit comprises a storage tank configured to store the BOG which is collected through the BOG collection pipe and compressed at a high pressure by the high-pressure compressor , wherein the BOG treatment unit comprises a propulsion engine , a boiler , a gas turbine or a generator of the bunkering ship , wherein the BOG treatment unit may comprise a vent for discarding BOG to the air or a gas burner for burning BOG, wherein the BOG treatment unit is configured to supply and to consume the BOG stored in the storage tank in the engine , the boiler , the gas turbine or the generator of the bunkering ship , wherein the BOG treatment unit comprises a reliquefaction unit configured to reliquefy BOG collected through the BOG collection pipe , and wherein the BOG compressed by the low-pressure compressor is supplied to an energy consumer and to the reliquefaction unit . a liquefied-fuel-gas storage tank installed in the bunkering ship ; a fuel supply pipe configured to be connected to the liquefied-fuel-gas storage tank and to supply fuel to the liquefied-fuel-gas propulsion ship ; and a boil-off gas (BOG) collection pipe configured to be connected to the liquefied-fuel-gas propulsion ship and to collect BOG produced in a liquefied fuel gas tank of the liquefied-fuel-gas propulsion ship while the liquefied-fuel-gas is supplied; wherein the bunkering ship further comprises a BOG treatment unit, wherein the BOG treatment unit is configured to treat the BOG collected through the BOG collection pipe , wherein the BOG treatment unit comprises a high-pressure compressor configured to compress the BOG collected through the BOG collection pipe to high pressure and a low-pressure compressor configured to compress the BOG collected through the BOG collection pipe to low pressure, wherein the BOG treatment unit comprises a storage tank configured to store the BOG which is collected through the BOG collection pipe and compressed at a high pressure by the high-pressure compressor , wherein the BOG treatment unit comprises a propulsion engine , a boiler , a gas turbine or a generator of the bunkering ship , wherein the BOG treatment unit may comprise a vent for discarding BOG to the air or a gas burner for burning BOG, wherein the BOG treatment unit is configured to supply and to consume the BOG stored in the storage tank in the engine , the boiler , the gas turbine or the generator of the bunkering ship , wherein the BOG treatment unit comprises a reliquefaction unit configured to reliquefy BOG collected through the BOG collection pipe , and wherein the BOG compressed by the low-pressure compressor is supplied to an energy consumer and to the reliquefaction unit . <EOS>