A gas supply arrangement of a marine vessel being adapted to carry liquefied gas in its cargo tank having an ullage space section and a liquid phase section, which arrangement utilizes the gas as fuel to provide power for the vessel, the arrangement comprising a first gas supply line provided for processing the natural boil-off gas formed in the cargo tank, a second gas supply line which connects the cargo tank and the gas main supply line and which is provided with at least a pump for raising the pressure of the liquid gas and for pumping it forward. The second gas supply line is provided with a gas reservoir having an ullage space section and liquid phase section, and that the arrangement is provided with a first duct section of the second gas supply line connecting the liquid phase section of the cargo tank and the liquid phase section of the gas reservoir, and being provided with the pump, and that the arrangement is additionally provided with a return line connecting the liquid phase section of the reservoir and the cargo tank being provided with a control valve for controllable returning liquid gas back into the cargo tank.