PURPOSE: To be able to reduce the energy required to vaporize liquefied gas by connecting first and second liquefied gas supply tubes for supplying liquefied gas to first and second vaporizing tanks to the liquefied gas supply main tube connected to a liquefied gas storage tank.

CONSTITUTION: First and second water supply tubes 31, 32 for supplying sea water to first and second vaporizing tanks 11, 12 are connected to a water supply main tube 23. When it is judged that the temperature of the sea water is sufficiently high and the water W of the quantity to be supplied to the first tube 31 is sufficient to vaporize the liquefied gas in both the first and second tanks 11, 12, the water W supplied to the tank 11 and heat exhausted with the liquefied gas to be vaporized is collected by a trough 14, and supplied to the tank 12. (Since it is again heat exchanged with the liquefied gas and vaporized, the quantity of the sea water to be supplied to the liquefied gas for vaporizing is reduced by half, and hence, the energy required to operate the pump is deleted.