Provided is a fuel supply method for a marine structure using a high-pressure natural gas injection engine. BOG stored in a stored in the storage tank is compressed to a pressure of 12 to 45 bara (absolute pressure) and then reliquefied. A reliquefaction apparatus includes a cold box configured to exchange heat between a refrigerant and the BOG, a compression unit configured to compress the refrigerant heated by the cold box, an expansion unit configured to expand the compressed refrigerant to drop the temperature thereof, and a plurality of gas-liquid refrigerant separators configured to separate the refrigerant into a gaseous refrigerant and a liquid refrigerant. A gaseous refrigerant and a liquid refrigerant separated by the gas-liquid refrigerant separator disposed at an upstream side are again mixed and supplied to the gas-liquid refrigerant separator disposed at the most downstream among the plurality of gas-liquid refrigerant separators.