TO BE SOLVED: To maintain a good operation by lowering a compressor operating temperature at the start and a temperature inside a heat container rapidly with simple and low cost structure in a device for processing BOG(Boil Off Gas) as well as LNG(Liquefied Natural Gas). SOLUTION: In a processing equipment, while LNG is fed by a heat container 30, BOG in an LNG reservoir 10 is force-fed to a heat exchanger 18 by a BOG compressor 16, heat-exchanged there with LNG and cooled, and mixed with LNG. It is provided with a circulating pipe 20 to circulate BOG from the downstream side of the heat exchanger 18 to the upstream side of the BOG compressor 16 and a temperature controller 24. The temperature controller 24 detects the BOG temperature at the upstream side of the BOG compressor 16, and adjusts the degree of the opening of a flow control valve 22 located in the circulating pipe 20 so that more BOG can be circulated as the detected temperature is more increased until the detected temperature is lowered to a specified target temperature.