PURPOSE: To improve economy of gas for treatment of the evaporated gas during transport of natural liquefied gas mainly composed of liquefied ethane, by not liquefiying gas mainly composed of methane of low liquefied temperature in a reliquefying process but separating, collecting and utilizing the gas as fuel for a mother ship.

CONSTITUTION: Evaporated gas generated in freight tanks 2 of a tanker 1 passes through a conduction pipe 3 and is fed to a compressor 4 and pressurized, and then guided to a condenser 6 through a conduction pipe 5, here gas of relatively high liquefying temperature mainly composed of ethane is condensed, liquefied and returned to the original tank 2 through a conduction pipe 7, but gas mainly composed of methane is not liquefied but used as in a state of gas for driving fuel of various engines 9 in a mother ship through a conduction pipe 8, and then discharged safely as carbon dioxide and vapor to the atmosphere through an exhaust pipe 10.