To prevent a liquefied gas from leaking from a pipe connected to a bottom of a tank, and to reduce costs.

A pipe connected to a bottom of a tank 20 is not installed. After a part of a liquefied gas is taken out from above the tank 20 by utilizing pressure in the tank 20 and a gas bottle 31 is filled with the part of the gas, gas fuel obtained by heating and vaporizing the liquefied gas in the gas bottle 31 is supplied into the tank 20 so that the inside of the tank 20 is pressurized to send out the liquefied gas. At this time, a plurality of sets of gas bottle units 30 each having the gas bottle 31 are provided to periodically delay operation timing.