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 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 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.