PROBLEM TO BE SOLVED: To synthesize dimethyl ether from a boil-off gas and produce a town gas by using dimethyl ether thus produced as a carbureting agent. SOLUTION: A boil-off gas sent from an LNG storage tank 21 is pressurized to 10-70 kg/cm with a compressor 3 and is synthesized into dimethyl ether in a production plant 41. Thus produced dimethyl ether is stored in a storage tank 42, then is vaporized with a vaporizer 43, and is supplied to a town gas line at a pressure of 1.3 times that of the town gas line in a vol. ratio to 19 vol, LNG of about 0.1-2. When the boil-off gas is insufficient, a part of LNG sent from the LNG storage tank 21 is vaporized with an auxiliary vaporizer 44 and is supplied as a supplement material to the dimethyl ether production plant 41.