A method for reliquefying gas at a vessel, the method comprising: bifurcating boil-off gas discharged from a storage tank into two flows in an initial stage of system operation and sending one of the two flows to a main compression unit while sending the other flow to an extra compression unit; joining the boil-off gas compressed by the main compression unit and the boil-off gas compressed by the extra compression unit with each other; supplying some of the boil-off gas compressed by the main compression unit and the boil-off gas compressed by the extra compression unit to a heat exchanger after the initial stage of system operation; sending boil-off gas discharged from the storage tank to the heat exchanger; bifurcating the boil-off gas discharged from the storage tank and having passed through the heat exchanger into two flows and sending one of the two flows to the main compression unit while sending the other flow to the extra compression unit; joining the boil-off gas compressed by the main compression unit and the boil-off gas compressed by the extra compression unit with each other, followed by sending part of the joined boil-off gas to an engine while sending the other part of the joined boil-off gas to the heat exchanger; reliquefying a fluid cooled in the heat exchanger through heat exchange with the boil-off gas discharged from the storage tank through expansion by a decompressor, and separating the reliquefied fluid into a gas phase and a liquid phase by a gas-liquid separator such that the liquefied gas is returned to the storage tank and the boil-off gas remaining in the gas phase is joined with the boil-off gas discharged from the storage tank to be sent to the heat exchanger, wherein, the main compression unit and the extra compression unit are operated at the same time when there is a need for rapid treatment of the boil-off gas immediately after navigation of the vessel or immediately before port entry. bifurcating boil-off gas discharged from a storage tank into two flows in an initial stage of system operation and sending one of the two flows to a main compression unit while sending the other flow to an extra compression unit; joining the boil-off gas compressed by the main compression unit and the boil-off gas compressed by the extra compression unit with each other; supplying some of the boil-off gas compressed by the main compression unit and the boil-off gas compressed by the extra compression unit to a heat exchanger after the initial stage of system operation; sending boil-off gas discharged from the storage tank to the heat exchanger; bifurcating the boil-off gas discharged from the storage tank and having passed through the heat exchanger into two flows and sending one of the two flows to the main compression unit while sending the other flow to the extra compression unit; joining the boil-off gas compressed by the main compression unit and the boil-off gas compressed by the extra compression unit with each other, followed by sending part of the joined boil-off gas to an engine while sending the other part of the joined boil-off gas to the heat exchanger; reliquefying a fluid cooled in the heat exchanger through heat exchange with the boil-off gas discharged from the storage tank through expansion by a decompressor, and separating the reliquefied fluid into a gas phase and a liquid phase by a gas-liquid separator such that the liquefied gas is returned to the storage tank and the boil-off gas remaining in the gas phase is joined with the boil-off gas discharged from the storage tank to be sent to the heat exchanger, wherein, the main compression unit and the extra compression unit are operated at the same time when there is a need for rapid treatment of the boil-off gas immediately after navigation of the vessel or immediately before port entry.