PURPOSE: To keep the internal pressure of a cold insulation tank at a constant pressure without causing the reduction in the heat insulating performance of the cold insulation tank by selectively supplying very low temperature fuel vaporized in a fuel supply pipe into a fuel reservoir or a cold insulation tank.

CONSTITUTION: A solenoid changeover valve 50 interposed between a fuel supply pipe 20 and a pressure-retaining pipe 40 is properly changed over on the basis of the pressure of LNG flowing in the fuel supply pipe 20, and the vaporized LNG is properly supplied into the inner tank 11 of a cold insulation tank 10, thereby the internal pressure of the cold insulation tank 10 is kept at a constant pressure.