PURPOSE: To enable a fuel liquid surface within a fixed tank to be always kept constant by a method wherein an oil supplying port is arrange at an upper part of a cartridge tank to be installed at the fixed tank and a pressurizing device is driven when a liquid surface sensor within the fixed tank reaches a predetermined liquid surface.

CONSTITUTION: A cartridge tank 3 has at its upper part an oil supplying port 13 for supplying fuel and an oil supplying port 14 for guiding fuel to a fixed tank 4. If fuel within a fixed tank 4 is decreased, a liquid surface (a) is lowered, a float 21 of a liquid surface switch 20 is lowered and a contact point of a lead switch is turned on. A driving circuit 23 may detect this condition and when a pressurizing device 16 is operated, a pressure within a cartridge tank 3 is increased through a pressurizing pipe 19 and a pressurizing port 17. With this operation, a liquid surface (b) is lowered and the fuel is pushed up through a suction pipe 18, the fuel is supplied to the fixed tank through an oil feeding port 14, an oil feeding pipe 27 and a limiting device 22, resulting in that the oil is fed while the liquid surface (a) of the fixed tank 4 being kept substantially constant.