To reliquefy BOG(boil off gas) even with a small quantity of LNG by utilizing cold heat of LNG efficiently.

BOG generated from an LNG tank 1 is boosted through a BOG compressor 6 and mixed with a part of LNG boosted through an LNG pump 2. The mixture fluid is precooled and the components thereof approaches to those of LNG. The mixture fluid and the LNG exchange heat through a BOG liquefier 4. Since the cooking curve of the mixture fluid is approximated to the heating curve of LNG, both fluids exchange heat efficiently over a wide temperature range and the LNG is heated up from -153°C to -73.7°C and evaporated and thereby latent heat as well as sensible heat can be taken out as cold heat. The mixture fluid of BOG and LNG is cooled down to -140°C and liquefied by reducing the pressure through a reducing valve 10 before being returned back to the LNG tank 1.