PURPOSE: To eliminate the necessity of a facility including a pressure boost pump and a mixer in a device for restraining BOG generated in a liquefied storage tank.

CONSTITUTION: Intermediate medium sealed in an LPG supercooling heatexchanger is cooled by LNG, and a part of LPG branching from an LPG suck-up line 2 is supercooled by the cold heat of the intermediate medium and is then returned into an LPG storage tank 1. Accordingly, the supercooled LPG can restrain the generation of BOG in the LPG storage tank 1.